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Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦...

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Unit WorkBook 4 – Level 4 ENG– U2 Engineering Maths Β© 2018 UniCourse Ltd. All Rights Reserved. 1 of 38 Pearson BTEC Higher Nationals in Electrical and Electronic Engineering (RQF) Unit 2: Engineering Maths (core) Unit Workbook 4 in a series of 4 for this unit Learning Outcome 4 Calculus Sample
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Page 1: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

Unit WorkBook 4 – Level 4 ENG– U2 Engineering Maths Β© 2018 UniCourse Ltd. All Rights Reserved.

1 of 38

Pearson BTEC Higher Nationals in Electrical and Electronic Engineering (RQF)

Unit 2: Engineering Maths (core)

Unit Workbook 4 in a series of 4 for this unit

Learning Outcome 4

Calculus

Sample

Page 2: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

Unit WorkBook 4 – Level 4 ENG– U2 Engineering Maths Β© 2018 UniCourse Ltd. All Rights Reserved.

4 of 38

3.1 Calculus There are a couple of great webpages for checking your answers to Calculus problems…

Check Differentiation answers

Check Integration answers

3.1.1 The Concept of the Limit, Continuity and the Derivative Calculus deals with functions which continually vary and is based upon the concept of a limit and continuity. Let’s refer to a diagram to understand the concept of a limit…

Here we have indicated a point P on part of a function (the red curve) with Cartesian co-ordinates (π‘₯π‘₯1,𝑦𝑦1). We require another point Q on the function to be a small increment away from P and will designate a small increment with the symbol 𝛿𝛿 (delta).

What we then have is…

𝑷𝑷 = (π’™π’™πŸπŸ,π’šπ’šπŸπŸ)

𝑸𝑸 = (π’™π’™πŸπŸ + πœΉπœΉπ’™π’™, π’šπ’šπŸπŸ + πœΉπœΉπ’šπ’š)

Look now at the chord PQ (drawn as the straight line in blue). If we can determine the slope of this chord and then make it infinitesimally short we will end up with a tangent to the function. It is the slope of this tangent which forms the basis of Differential Calculus (normally called differentiation).

By inspection, we see that the slope of the chord is given by…

𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔𝒔 𝒔𝒔𝒐𝒐 𝒄𝒄𝒄𝒄𝒔𝒔𝒄𝒄𝒄𝒄 𝑷𝑷𝑸𝑸 =πœΉπœΉπ’šπ’šπœΉπœΉπ’™π’™

If we deliberately make 𝛿𝛿π‘₯π‘₯ approach zero (i.e. make it as short as possible) then we shall reach a limit, which may expressed mathematically as…

Sample

Page 3: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

Unit WorkBook 4 – Level 4 ENG– U2 Engineering Maths Β© 2018 UniCourse Ltd. All Rights Reserved.

5 of 38

Worked Example 1

π’„π’„π’šπ’šπ’„π’„π’™π’™

= πœΉπœΉπ’™π’™π’”π’”π’π’π’π’π’π’π’π’οΏ½βŽ―βŽ―οΏ½ 𝟎𝟎

πœΉπœΉπ’šπ’šπœΉπœΉπ’™π’™

The term 𝑑𝑑𝑦𝑦 𝑑𝑑π‘₯π‘₯⁄ is written in Leibnitz notation and indicates the slope of the chord when the chord only touches the function at one single point. This is achieved by continual reduction of 𝛿𝛿π‘₯π‘₯.

What we can now say is that we are able to find the slope of any function by adopting this process. Hence, given a function 𝑓𝑓(π‘₯π‘₯) we are able to differentiate that function, meaning find its slope at all points. We can write…

The slope at any point of a function 𝑓𝑓(π‘₯π‘₯) is given by 𝒄𝒄(𝒐𝒐(𝒙𝒙))𝒄𝒄𝒙𝒙

This process is called finding the derivative of a function.

3.1.2 Derivatives of Standard Functions What we don’t want to be doing is to spend too much time drawing graphs of functions just to work out the derivative. Fortunately there are standard ways to determine the derivative of functions and some of the frequent ones which engineers meet are given in the table below.

Function Derivative 𝐴𝐴π‘₯π‘₯𝑛𝑛 𝑛𝑛𝐴𝐴π‘₯π‘₯π‘›π‘›βˆ’1

A sin (π‘₯π‘₯) A cos (π‘₯π‘₯)

A cos (π‘₯π‘₯) βˆ’π΄π΄ sin (π‘₯π‘₯)

𝐴𝐴 π‘’π‘’π‘˜π‘˜π‘˜π‘˜ π‘˜π‘˜π΄π΄π‘’π‘’π‘˜π‘˜π‘˜π‘˜

𝐴𝐴 𝑙𝑙𝑙𝑙𝑙𝑙𝑒𝑒(π‘₯π‘₯) 𝐴𝐴 π‘₯π‘₯οΏ½

A sinh (π‘₯π‘₯) A cosh (π‘₯π‘₯)

A cosh (π‘₯π‘₯) A sinh (π‘₯π‘₯)

Let’s look at some examples of using these standard derivatives…

Differentiate the function π’šπ’š = πŸ‘πŸ‘π’™π’™πŸ’πŸ’ with respect to 𝒙𝒙.

We see that this function is similar to that in row 1 of our table. We can see that 𝐴𝐴 = 3 and 𝑛𝑛 = 4. We may then write…

Sample

Page 4: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

Unit WorkBook 4 – Level 4 ENG– U2 Engineering Maths Β© 2018 UniCourse Ltd. All Rights Reserved.

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Worked Example 2

Worked Example 3

Worked Example 4

𝑑𝑑𝑦𝑦𝑑𝑑π‘₯π‘₯

= 𝑛𝑛𝐴𝐴π‘₯π‘₯π‘›π‘›βˆ’1 = (4)(3)π‘₯π‘₯4βˆ’1 = 12π‘₯π‘₯3

Differentiate the function π’šπ’š = πŸ”πŸ”π’™π’™βˆ’πŸ“πŸ“ with respect to 𝒙𝒙.

We see that this function is similar to that in row 1 of our table. We can see that 𝐴𝐴 = 6 and 𝑛𝑛 = βˆ’5. We may then write…

𝑑𝑑𝑦𝑦𝑑𝑑π‘₯π‘₯

= 𝑛𝑛𝐴𝐴π‘₯π‘₯π‘›π‘›βˆ’1 = (βˆ’5)(6)π‘₯π‘₯βˆ’5βˆ’1 = βˆ’30π‘₯π‘₯βˆ’6

Differentiate the function 𝒗𝒗 = 𝟏𝟏𝟏𝟏 𝒔𝒔𝒍𝒍𝒔𝒔(𝒍𝒍) with respect to 𝒍𝒍.

Don’t worry that 𝑦𝑦 and π‘₯π‘₯ have disappeared here. We can use any letters we like. The letter 𝑣𝑣 means voltage and 𝑑𝑑 is time. All we want to do here then is to find 𝑑𝑑𝑣𝑣 𝑑𝑑𝑑𝑑� .

We see that the function is similar to that in row 2 of our table. We may write…

𝑑𝑑𝑣𝑣𝑑𝑑𝑑𝑑

= 𝐴𝐴 cos(𝑑𝑑) = 12 cos (𝑑𝑑)

Differentiate the function 𝒍𝒍 = πŸ“πŸ“ 𝒄𝒄𝒔𝒔𝒔𝒔(𝒍𝒍) with respect to 𝒍𝒍.

The letter 𝑖𝑖 means current and 𝑑𝑑 is time. All we want to do here then is to find 𝑑𝑑𝑖𝑖 𝑑𝑑𝑑𝑑� .

We see that the function is similar to that in row 3 of our table. We may write…

𝑑𝑑𝑖𝑖𝑑𝑑𝑑𝑑

= βˆ’π΄π΄ sin(𝑑𝑑) = βˆ’5 sin (𝑑𝑑)

Sample

Page 5: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

Unit WorkBook 4 – Level 4 ENG– U2 Engineering Maths Β© 2018 UniCourse Ltd. All Rights Reserved.

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Worked Example 5

Worked Example 6

Worked Example 7

Worked Example 8

Questions

Differentiate the function π’šπ’š = πŸ’πŸ’π’”π’”πŸπŸπ’™π’™ with respect to 𝒙𝒙.

From row 4 of our table we can write…

𝑑𝑑𝑦𝑦𝑑𝑑π‘₯π‘₯

= π‘˜π‘˜π΄π΄π‘’π‘’π‘˜π‘˜π‘˜π‘˜ = (2)(4)𝑒𝑒2π‘˜π‘˜ = 8𝑒𝑒2π‘˜π‘˜

Differentiate the function π’šπ’š = πŸπŸπŸ”πŸ” 𝒔𝒔𝒔𝒔𝒍𝒍𝒔𝒔(𝒙𝒙) with respect to 𝒙𝒙.

We refer to row 5 of our table and write…

𝑑𝑑𝑦𝑦𝑑𝑑π‘₯π‘₯

=𝐴𝐴π‘₯π‘₯

=16π‘₯π‘₯

Differentiate the function π’šπ’š = πŸ—πŸ— 𝒔𝒔𝒍𝒍𝒔𝒔𝒄𝒄(𝒙𝒙) with respect to 𝒙𝒙.

We refer to row 6 of our table and write…

𝑑𝑑𝑦𝑦𝑑𝑑π‘₯π‘₯

= 𝐴𝐴 cosh(π‘₯π‘₯) = 9 cosh (π‘₯π‘₯)

Differentiate the function π’šπ’š = βˆ’πŸπŸπŸ’πŸ’.πŸ“πŸ“ 𝒄𝒄𝒔𝒔𝒔𝒔𝒄𝒄(𝒙𝒙) with respect to 𝒙𝒙.

We refer to row 7 of our table and write…

𝑑𝑑𝑦𝑦𝑑𝑑π‘₯π‘₯

= 𝐴𝐴 sinh(π‘₯π‘₯) = βˆ’14.5 sinh (π‘₯π‘₯)

Q3.1 Differentiate the function π’šπ’š = πŸ–πŸ–π’™π’™πŸ“πŸ“ with respect to 𝒙𝒙.

Sample

Page 6: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

Unit WorkBook 4 – Level 4 ENG– U2 Engineering Maths Β© 2018 UniCourse Ltd. All Rights Reserved.

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Video

Q3.2 Differentiate the function π’šπ’š = πŸ•πŸ•π’™π’™βˆ’πŸ’πŸ’ with respect to 𝒙𝒙.

Q3.3 Differentiate the function 𝒗𝒗 = πŸπŸπŸ–πŸ– 𝒔𝒔𝒍𝒍𝒔𝒔(𝒍𝒍) with respect to 𝒍𝒍.

Q3.4 Differentiate the function 𝒍𝒍 = 𝟏𝟏𝟏𝟏 𝒄𝒄𝒔𝒔𝒔𝒔(𝒍𝒍) with respect to 𝒍𝒍.

Q3.5 Differentiate the function π’šπ’š = πŸ”πŸ”π’”π’”πŸ‘πŸ‘π’™π’™ with respect to 𝒙𝒙.

Q3.6 Differentiate the function π’šπ’š = 𝟏𝟏𝟎𝟎 𝒔𝒔𝒔𝒔𝒍𝒍𝒔𝒔(𝒙𝒙) with respect to 𝒙𝒙.

Q3.7 Differentiate the function π’šπ’š = πŸπŸπŸ‘πŸ‘ 𝒔𝒔𝒍𝒍𝒔𝒔𝒄𝒄(𝒙𝒙) with respect to 𝒙𝒙.

Q3.8 Differentiate the function π’šπ’š = βˆ’πŸπŸπŸ—πŸ—.πŸ”πŸ”πŸπŸ 𝒄𝒄𝒔𝒔𝒔𝒔𝒄𝒄(𝒙𝒙) with respect to 𝒙𝒙.

ANSWERS

These videos will boost your knowledge of differentiation

3.1.3 Notion of the Derivative and Rates of Change The notion of taking the derivative of a function and examining rate of change can be readily explained with reference to the diagram below…

Q3.1 40π‘₯π‘₯4

Q3.2 βˆ’28π‘₯π‘₯βˆ’5

Q3.3 18 cos(𝑑𝑑)

Q3.4 βˆ’11 sin(𝑑𝑑)

Q3.5 18𝑒𝑒3π‘˜π‘˜

Q3.6 20 π‘₯π‘₯⁄

Q3.7 13 cosh(π‘₯π‘₯)

Q3.8 βˆ’19.62 sinh(π‘₯π‘₯) Sample

Page 7: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

Unit WorkBook 4 – Level 4 ENG– U2 Engineering Maths Β© 2018 UniCourse Ltd. All Rights Reserved.

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Challenge

Here we have plotted a section of a sine wave in red. Notice that we have used pink arrows to indicate the slope at various points on the sine wave. For example, at the start the sine wave is rising to a positive more rapidly than at any other point. The pink arrow indicates this positive growth by pointing upwards in the direction of the sine wave at that beginning point. The second arrow shows the slope of the sine wave at πœ‹πœ‹ 2⁄ radians (or 900 if you like). Here it is at a plateau and has zero slope (meaning that it just moves horizontally, with no vertical movement at all). The third arrow indicates a negative slope (its moving down very steeply). The picture continues in a similar way for the remaining arrows.

Here’s the really interesting bit. When we record all of the slopes on our sine wave we get the dotted waveform in blue. What does that look like? Of course, it is a cosine wave. So we have just proved graphically that finding the slope (differentiating) at all points on a sine wave leads to a cosine wave.

∴ 𝒄𝒄{𝐬𝐬𝐬𝐬𝐬𝐬 (𝒍𝒍)}

𝒄𝒄𝒍𝒍= 𝐜𝐜𝐜𝐜𝐬𝐬 (𝒍𝒍)

… as given in our table of standard derivatives in section 3.1.2.

If we had loads of time to kill we could prove all of the other derivatives graphically.

Determine a graphical proof (as above) for…

𝒄𝒄{𝐜𝐜𝐜𝐜𝐬𝐬 (𝒍𝒍)}𝒄𝒄𝒍𝒍

= βˆ’π¬π¬π¬π¬π¬π¬ (𝒍𝒍)

Sample

Page 8: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

Unit WorkBook 4 – Level 4 ENG– U2 Engineering Maths Β© 2018 UniCourse Ltd. All Rights Reserved.

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3.2.3 Differentiation of Inverse Trigonometric Functions Suppose we have the equation…

𝑦𝑦 = π΄π΄π‘–π‘–π‘›π‘›βˆ’1(π‘₯π‘₯)

How do we then go about finding 𝑑𝑑𝑦𝑦 𝑑𝑑π‘₯π‘₯⁄ ? Maybe we should write this in a different form…

π‘₯π‘₯ = sin(𝑦𝑦)

Now can differentiate…

𝑑𝑑π‘₯π‘₯𝑑𝑑𝑦𝑦

= cos(𝑦𝑦) ∴ 𝑓𝑓𝑙𝑙𝑖𝑖𝑓𝑓 π‘ π‘ π‘™π‘™π‘‘π‘‘β„Ž 𝐴𝐴𝑖𝑖𝑑𝑑𝑒𝑒𝐴𝐴: 𝑑𝑑𝑦𝑦𝑑𝑑π‘₯π‘₯

=1

cos (𝑦𝑦)

Next we shall remember one of our handy trig. Identities…

𝑐𝑐𝑙𝑙𝐴𝐴2(𝑦𝑦) + 𝐴𝐴𝑖𝑖𝑛𝑛2(𝑦𝑦) = 1 ∴ 𝑐𝑐𝑙𝑙𝐴𝐴2(𝑦𝑦) = 1 βˆ’ 𝐴𝐴𝑖𝑖𝑛𝑛2(𝑦𝑦) ∴ cos(𝑦𝑦) = οΏ½1 βˆ’ 𝐴𝐴𝑖𝑖𝑛𝑛2(𝑦𝑦)

Since we know π‘₯π‘₯ = sin(𝑦𝑦) then if we square both sides we shall have π‘₯π‘₯2 = 𝐴𝐴𝑖𝑖𝑛𝑛2(𝑦𝑦)

So we may re-write our transposed trig. identity as…

cos(𝑦𝑦) = οΏ½1 βˆ’ π‘₯π‘₯2

Now we can put this latest result back into our differential…

π’„π’„π’šπ’šπ’„π’„π’™π’™

=𝟏𝟏

𝐜𝐜𝐜𝐜𝐬𝐬 (π’šπ’š)=

𝟏𝟏√𝟏𝟏 βˆ’ π’™π’™πŸπŸ

=𝟏𝟏

(𝟏𝟏 βˆ’ π’™π’™πŸπŸ)𝟎𝟎.πŸ“πŸ“ = (𝟏𝟏 βˆ’ π’™π’™πŸπŸ)βˆ’πŸŽπŸŽ.πŸ“πŸ“

That was a bit tricky, but we managed to find 𝑦𝑦′.

Let’s go on and see if we can find 𝑦𝑦′′…

𝑦𝑦′ = (1 βˆ’ π‘₯π‘₯2)βˆ’0.5

This is where that shorthand notation for derivatives comes in really handy…

𝐿𝐿𝑒𝑒𝑑𝑑 𝑒𝑒 = 1 βˆ’ π‘₯π‘₯2 ∴ 𝑑𝑑𝑒𝑒𝑑𝑑π‘₯π‘₯

= βˆ’2π‘₯π‘₯

∴ 𝑦𝑦′ = π‘’π‘’βˆ’0.5

∴ 𝑑𝑑𝑦𝑦′

𝑑𝑑𝑒𝑒= βˆ’0.5π‘’π‘’βˆ’1.5

∴ 𝑦𝑦′′ =𝑑𝑑𝑒𝑒𝑑𝑑π‘₯π‘₯

βˆ™ 𝑑𝑑𝑦𝑦′

𝑑𝑑𝑒𝑒= βˆ’2π‘₯π‘₯ Γ— βˆ’0.5π‘’π‘’βˆ’1.5

∴ π’šπ’šβ€²β€² = π’™π’™π’…π’…βˆ’πŸπŸ.πŸ“πŸ“ = 𝒙𝒙(𝟏𝟏 βˆ’ π’™π’™πŸπŸ)βˆ’πŸπŸ.πŸ“πŸ“

Sample

Page 9: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

Unit WorkBook 4 – Level 4 ENG– U2 Engineering Maths Β© 2018 UniCourse Ltd. All Rights Reserved.

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Question

Question

Q3.23 Find π’šπ’šβ€²β€² for π’šπ’š = π’„π’„π’”π’”π’”π’”βˆ’πŸπŸ(πŸπŸπ’™π’™)

ANSWER

3.2.4 Differentiation of Inverse Hyperbolic Functions Consider the function…

𝑦𝑦 = π΄π΄π‘–π‘–π‘›π‘›β„Žβˆ’1(π‘₯π‘₯)

Our task here is to find 𝑦𝑦′′. We proceed in a similar manner to the previous problem…

π‘₯π‘₯ = sinh (𝑦𝑦)

∴ 𝑑𝑑π‘₯π‘₯𝑑𝑑𝑦𝑦

= cosh(𝑦𝑦) ∴ 𝑑𝑑𝑦𝑦𝑑𝑑π‘₯π‘₯

=1

cosh (𝑦𝑦)

A useful identity is…

π‘π‘π‘™π‘™π΄π΄β„Ž2(𝑦𝑦)βˆ’ π΄π΄π‘–π‘–π‘›π‘›β„Ž2(𝑦𝑦) = 1 ∴ π‘π‘π‘™π‘™π΄π΄β„Ž2(𝑦𝑦) = 1 + π΄π΄π‘–π‘–π‘›π‘›β„Ž2(𝑦𝑦)

∴ π‘π‘π‘™π‘™π΄π΄β„Ž2(𝑦𝑦) = 1 + π‘₯π‘₯2 ∴ cosh(𝑦𝑦) = οΏ½1 + π‘₯π‘₯2

∴ π’„π’„π’šπ’šπ’„π’„π’™π’™

= π’šπ’šβ€² =𝟏𝟏

√𝟏𝟏 + π’™π’™πŸπŸ= (𝟏𝟏 + π’™π’™πŸπŸ)βˆ’πŸŽπŸŽ.πŸ“πŸ“

Now to find that second derivative…

𝑦𝑦′ = (1 + π‘₯π‘₯2)βˆ’0.5

𝐿𝐿𝑒𝑒𝑑𝑑 𝑒𝑒 = 1 + π‘₯π‘₯2 ∴ 𝑑𝑑𝑒𝑒𝑑𝑑π‘₯π‘₯

= 2π‘₯π‘₯

∴ 𝑦𝑦′ = π‘’π‘’βˆ’0.5 ∴ 𝑑𝑑𝑦𝑦′

𝑑𝑑𝑒𝑒= βˆ’0.5π‘’π‘’βˆ’1.5

∴ 𝑦𝑦′′ =𝑑𝑑𝑒𝑒𝑑𝑑π‘₯π‘₯

βˆ™ 𝑑𝑑𝑦𝑦′

𝑑𝑑𝑒𝑒= 2π‘₯π‘₯ Γ— βˆ’0.5π‘’π‘’βˆ’1.5 = βˆ’π‘₯π‘₯(1 + π‘₯π‘₯2)βˆ’1.5

Q3.24 Find π’šπ’šβ€²β€² for π’šπ’š = π’„π’„π’”π’”π’”π’”π’„π’„βˆ’πŸπŸ(πŸπŸπ’™π’™)

ANSWER

Q3.23 βˆ’8π‘˜π‘˜(1βˆ’4π‘˜π‘˜2)1.5

Sample

Page 10: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

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Worked Example 22

3.3 Further Integration

3.3.1 Integration by Parts This technique provides us with a way to integrate the product of two simple functions. We shall develop a formula to use as a framework for the technique.

The starting point for our formula lies with the Product Rule, which you mastered in Section 3.1.5…

𝑑𝑑𝑑𝑑π‘₯π‘₯

(𝑒𝑒𝑣𝑣) = 𝑒𝑒𝑑𝑑𝑣𝑣𝑑𝑑π‘₯π‘₯

+ 𝑣𝑣𝑑𝑑𝑒𝑒𝑑𝑑π‘₯π‘₯

Let’s rearrange this formula…

𝑒𝑒𝑑𝑑𝑣𝑣𝑑𝑑π‘₯π‘₯

=𝑑𝑑𝑑𝑑π‘₯π‘₯

(𝑒𝑒𝑣𝑣) βˆ’ 𝑣𝑣𝑑𝑑𝑒𝑒𝑑𝑑π‘₯π‘₯

If we now integrate both sides with respect to π‘₯π‘₯ then we need to place a ∫ sign before each term and a 𝑑𝑑π‘₯π‘₯ at the end of each term, like so…

�𝑒𝑒𝑑𝑑𝑣𝑣𝑑𝑑π‘₯π‘₯

𝑑𝑑π‘₯π‘₯ = �𝑑𝑑𝑑𝑑π‘₯π‘₯

(𝑒𝑒𝑣𝑣)𝑑𝑑π‘₯π‘₯ βˆ’οΏ½π‘£π‘£π‘‘π‘‘π‘’π‘’π‘‘π‘‘π‘₯π‘₯

𝑑𝑑π‘₯π‘₯

Some simplifications can be noticed here:

In the first integral the two 𝑑𝑑π‘₯π‘₯ terms cancel each other out In the second integral we are taking the β€˜integral of the differential’ of 𝑒𝑒𝑣𝑣. Since integration is the

reverse of differentiation then the ’integral of a differential’ drops away, just leaving 𝑒𝑒𝑣𝑣 In the third integral the two 𝑑𝑑π‘₯π‘₯ terms cancel each other out

Performing these simplifications gives…

�𝒅𝒅 𝒄𝒄𝒗𝒗 = 𝒅𝒅𝒗𝒗 βˆ’οΏ½π’—π’— 𝒄𝒄𝒅𝒅

That is our formula for integration by parts.

What we do with the formula is to look at the left hand side and see the integral of a product. That product is composed of 𝑒𝑒 and 𝑑𝑑𝑣𝑣. When using integration by parts you have a choice of which part of the product to call 𝑒𝑒 and which to call 𝑑𝑑𝑣𝑣. Fortunately, your choice is guided…

The 𝑒𝑒 part will become a constant after taking multiple derivatives The 𝑑𝑑𝑣𝑣 part is readily integrated by using standard integrals.

Sample

Page 11: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

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Worked Example 23

Determine: βˆ«π’™π’™ 𝐬𝐬𝐬𝐬𝐬𝐬(𝒙𝒙)𝒄𝒄𝒙𝒙

One part of our product is π‘₯π‘₯ and the other part is sin (π‘₯π‘₯). We need to make a choice as to which one to make 𝑒𝑒 and which to make 𝑑𝑑𝑣𝑣. Our guidance tells us to make the π‘₯π‘₯ part equal to 𝑒𝑒 since the derivative of π‘₯π‘₯ becomes 1 (i.e. a constant)…

𝐿𝐿𝑒𝑒𝑑𝑑 𝑒𝑒 = π‘₯π‘₯

𝐿𝐿𝑒𝑒𝑑𝑑 𝑑𝑑𝑣𝑣 = sin (π‘₯π‘₯)

We now try to form the terms in our integration by parts formula…

𝐼𝐼𝑓𝑓 𝑒𝑒 = π‘₯π‘₯ ∴ 𝑑𝑑𝑒𝑒𝑑𝑑π‘₯π‘₯

= 1 ∴ 𝑑𝑑𝑒𝑒 = 𝑑𝑑π‘₯π‘₯

𝐼𝐼𝑓𝑓 𝑑𝑑𝑣𝑣 = sin(π‘₯π‘₯) ∴ 𝑣𝑣 = βˆ’ cos (π‘₯π‘₯)

We can now write…

�𝒅𝒅 𝒄𝒄𝒗𝒗 = 𝒅𝒅𝒗𝒗 βˆ’οΏ½π’—π’— 𝒄𝒄𝒅𝒅

οΏ½π‘₯π‘₯ sin(π‘₯π‘₯)𝑑𝑑π‘₯π‘₯ = π‘₯π‘₯(βˆ’ cos(π‘₯π‘₯)) βˆ’οΏ½(βˆ’ cos(π‘₯π‘₯))𝑑𝑑π‘₯π‘₯

∴ οΏ½π‘₯π‘₯ sin(π‘₯π‘₯)𝑑𝑑π‘₯π‘₯ = βˆ’π‘₯π‘₯ cos(π‘₯π‘₯) + οΏ½ cos(π‘₯π‘₯)𝑑𝑑π‘₯π‘₯

∴ �𝒙𝒙𝐬𝐬𝐬𝐬𝐬𝐬(𝒙𝒙)𝒄𝒄𝒙𝒙 = βˆ’π’™π’™πœπœπœπœπ¬π¬(𝒙𝒙) + 𝐬𝐬𝐬𝐬𝐬𝐬(𝒙𝒙) + π‘ͺπ‘ͺ

That was fairly straightforward. Always remember the constant of integration 𝐢𝐢 at the end.

Let’s now try to integrate the product of an exponential and a cosine.

Determine: βˆ«π’”π’”πŸπŸπ’™π’™ 𝐜𝐜𝐜𝐜𝐬𝐬(𝒙𝒙)𝒄𝒄𝒙𝒙

Our two products here are 𝑒𝑒2π‘˜π‘˜ and cos (π‘₯π‘₯). Neither of them will reduce to a constant when differentiated so it doesn’t matter which one we call 𝑒𝑒 and which we call 𝑑𝑑𝑣𝑣. You will see here that we need to do integration by parts twice to arrive at our answer.

Just as in the previous worked example we need to decide on assignments for 𝑒𝑒 and 𝑑𝑑𝑣𝑣, then find 𝑣𝑣 and 𝑑𝑑𝑒𝑒. Let’s do that…

𝐿𝐿𝑒𝑒𝑑𝑑 𝑒𝑒 = 𝑒𝑒2π‘˜π‘˜ ∴ 𝑑𝑑𝑒𝑒𝑑𝑑π‘₯π‘₯

= 2𝑒𝑒2π‘˜π‘˜ ∴ 𝑑𝑑𝑒𝑒 = 2𝑒𝑒2π‘˜π‘˜π‘‘π‘‘π‘₯π‘₯

Sample

Page 12: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

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Question

𝐿𝐿𝑒𝑒𝑑𝑑 𝑑𝑑𝑣𝑣 = cos(π‘₯π‘₯) ∴ 𝑣𝑣 = sin (π‘₯π‘₯)

To aid our working here (and save some ink) we normally assign the capital letter 𝐼𝐼 to the integral in our question.

∴ 𝐼𝐼 = �𝑒𝑒2π‘˜π‘˜ cos(π‘₯π‘₯)𝑑𝑑π‘₯π‘₯

∴ 𝐼𝐼 = 𝑒𝑒2π‘˜π‘˜ sin(π‘₯π‘₯) βˆ’οΏ½ sin(π‘₯π‘₯) 2𝑒𝑒2π‘˜π‘˜π‘‘π‘‘π‘₯π‘₯

This needs tidying…

∴ 𝐼𝐼 = 𝑒𝑒2π‘˜π‘˜ sin(π‘₯π‘₯) βˆ’ 2�𝑒𝑒2π‘˜π‘˜sin(π‘₯π‘₯)𝑑𝑑π‘₯π‘₯

We were rather hoping for an answer, but what we seem to have is another β€˜integration by parts’ problem embedded into our development. Don’t worry, that’s quite normal for this type of problem. All we do is attack that last part with β€˜integration by parts’ once more, using square brackets. Our desired result will then appear…

𝐼𝐼 = 𝑒𝑒2π‘˜π‘˜ sin(π‘₯π‘₯) βˆ’ 2 �𝑒𝑒2π‘˜π‘˜(βˆ’cos (π‘₯π‘₯)) βˆ’οΏ½βˆ’cos (π‘₯π‘₯)2𝑒𝑒2π‘˜π‘˜π‘‘π‘‘π‘₯π‘₯οΏ½

Expanding these terms gives…

𝐼𝐼 = 𝑒𝑒2π‘˜π‘˜ sin(π‘₯π‘₯) + 2𝑒𝑒2π‘˜π‘˜ cos(π‘₯π‘₯) βˆ’ 4�𝑒𝑒2π‘˜π‘˜ cos(π‘₯π‘₯)𝑑𝑑π‘₯π‘₯

That last integral is the same as 𝐼𝐼, which is really handy, so we may now write…

𝐼𝐼 = 𝑒𝑒2π‘˜π‘˜ sin(π‘₯π‘₯) + 2𝑒𝑒2π‘˜π‘˜ cos(π‘₯π‘₯) βˆ’ 4𝐼𝐼

∴ 5𝐼𝐼 = 𝑒𝑒2π‘˜π‘˜ sin(π‘₯π‘₯) + 2𝑒𝑒2π‘˜π‘˜cos (π‘₯π‘₯)

If we divide both sides by 5 then we have our final answer…

𝑰𝑰 =π’”π’”πŸπŸπ’™π’™ 𝐬𝐬𝐬𝐬𝐬𝐬(𝒙𝒙) + πŸπŸπ’”π’”πŸπŸπ’™π’™πœπœπœπœπ¬π¬ (𝒙𝒙)

πŸ“πŸ“

Q3.25 Determine: βˆ«π’”π’”πŸ’πŸ’π’™π’™π’”π’”π’π’π’”π’”(πŸπŸπ’™π’™)𝒄𝒄𝒙𝒙

ANSWER

Q3.25 4𝑒𝑒4π‘₯π‘₯ sin(2π‘˜π‘˜)βˆ’2𝑒𝑒4π‘₯π‘₯cos(2π‘˜π‘˜)

20

Sample

Page 13: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

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3.4 Solution of Engineering Problems with Calculus

3.4.1 Charging RC Circuit For the circuit below, use Calculus to find the charge stored in the capacitor 2 seconds after the switch closes. Assume zero charge on the capacitor before this event.

For a capacitor charging via a DC source voltage, through a series resistor, the formula for instantaneous current is…

𝑖𝑖 =πΈπΈπ‘…π‘…βˆ™ π‘’π‘’οΏ½βˆ’π‘‘π‘‘ 𝑅𝑅𝑅𝑅� οΏ½ 𝐴𝐴𝑠𝑠𝑓𝑓𝐴𝐴

Since current is the defined as the rate of change of charge (𝑠𝑠, carried by passing electrons) we may say…

𝑖𝑖 =𝑑𝑑𝑠𝑠𝑑𝑑𝑑𝑑

If we integrate both sides with respect to 𝑑𝑑 we will have…

οΏ½ 𝑖𝑖 𝑑𝑑𝑑𝑑 = �𝑑𝑑𝑠𝑠𝑑𝑑𝑑𝑑

𝑑𝑑𝑑𝑑 = �𝑑𝑑𝑠𝑠 = 𝑠𝑠

So we may say that…

𝑠𝑠 = οΏ½ 𝑖𝑖 𝑑𝑑𝑑𝑑

If we wish to find the amount of charge store stored within a certain time period (i.e. 2 seconds in this example) we write…

𝑠𝑠(0βˆ’2) = οΏ½ 𝑖𝑖 𝑑𝑑𝑑𝑑2

0

= οΏ½πΈπΈπ‘…π‘…βˆ™ π‘’π‘’οΏ½βˆ’π‘‘π‘‘ 𝑅𝑅𝑅𝑅� οΏ½

2

0

𝑑𝑑𝑑𝑑

=πΈπΈπ‘…π‘…οΏ½π‘’π‘’οΏ½βˆ’1𝑅𝑅𝑅𝑅�𝑑𝑑

βˆ’1𝑅𝑅𝐢𝐢�

οΏ½

0

2

=βˆ’πΈπΈπ‘…π‘…πΆπΆπ‘…π‘…

οΏ½π‘’π‘’οΏ½βˆ’π‘‘π‘‘ 𝑅𝑅𝑅𝑅� οΏ½οΏ½0

2= βˆ’πΈπΈπΆπΆ οΏ½π‘’π‘’οΏ½βˆ’π‘‘π‘‘ 𝑅𝑅𝑅𝑅� οΏ½οΏ½

0

2

Since we know that 𝑅𝑅 = 1𝑀𝑀Ω and 𝐢𝐢 = 1πœ‡πœ‡πΉπΉ then 𝑅𝑅𝐢𝐢 = 1 Γ— 106 Γ— 1 Γ— 10βˆ’6 = 106βˆ’6 = 100 = 1 we may write…

Sample

Page 14: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

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𝑠𝑠(0βˆ’2) = βˆ’πΈπΈπΆπΆ οΏ½π‘’π‘’οΏ½βˆ’π‘‘π‘‘ 1οΏ½ οΏ½οΏ½0

2= βˆ’πΈπΈπΆπΆ[π‘’π‘’βˆ’π‘‘π‘‘]02 = βˆ’πΈπΈπΆπΆ[π‘’π‘’βˆ’2 βˆ’ π‘’π‘’βˆ’0] = βˆ’πΈπΈπΆπΆ[0.135 βˆ’ 1]

We also know that the DC supply voltage (𝐸𝐸) is 10V…

∴ 𝒒𝒒(πŸŽπŸŽβˆ’πŸπŸ) = βˆ’πŸπŸπŸŽπŸŽ Γ— 𝟏𝟏 Γ— πŸπŸπŸŽπŸŽβˆ’πŸ”πŸ”[βˆ’πŸŽπŸŽ.πŸ–πŸ–πŸ”πŸ”πŸ“πŸ“] = πŸ–πŸ–.πŸ”πŸ”πŸ“πŸ“ Γ— πŸπŸπŸŽπŸŽβˆ’πŸ”πŸ” π‘ͺπ‘ͺ𝒔𝒔𝒅𝒅𝒔𝒔𝒔𝒔𝒍𝒍π‘ͺπ‘ͺ𝒔𝒔 = πŸ–πŸ–.πŸ”πŸ”πŸ“πŸ“ 𝝁𝝁π‘ͺπ‘ͺ

The plot below illustrates the integration we have just performed…

Current is on the vertical axis and time on the horizontal. When we multiply current by time we get charge, which is shown as the shaded area between 0 and 2 seconds. You may like to experiment with the Graph simulator to produce similar results.

3.4.2 Energising Inductor Consider the series RL circuit below…

If 𝑬𝑬 = 𝟏𝟏𝟎𝟎 𝑽𝑽, 𝑹𝑹 = πŸπŸπŸπŸπ›€π›€ and 𝑳𝑳 = πŸπŸπŸŽπŸŽπŸŽπŸŽπ’π’πŸπŸ, use Calculus to determine the voltage across the inductor after 𝟎𝟎.πŸπŸπππ’”π’”. Use the following formulae in your solution development…

𝒍𝒍 =π‘¬π‘¬π‘Ήπ‘ΉοΏ½πŸπŸ βˆ’ π’”π’”βˆ’π‘Ήπ‘Ήπ’π’ 𝑳𝑳� οΏ½ 𝒂𝒂𝒔𝒔𝒄𝒄 𝒗𝒗𝑳𝑳 = 𝑳𝑳

𝒄𝒄𝒍𝒍𝒄𝒄𝒍𝒍

Sample

Page 15: Unit Workbook 4 Calculus - WB 4.pdfLet’s go on and see if we can find 𝑦𝑦 β€²β€² … 𝑦𝑦 β€² = (1 βˆ’π‘₯π‘₯ 2) βˆ’0.5 This is where that shorthand notation for derivatives

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We start with…

𝑣𝑣𝐿𝐿 = 𝐿𝐿𝑑𝑑𝑖𝑖𝑑𝑑𝑑𝑑

= 𝐿𝐿𝑑𝑑 �𝐸𝐸𝑅𝑅 οΏ½1 βˆ’ π‘’π‘’βˆ’π‘…π‘…π‘‘π‘‘ 𝐿𝐿� οΏ½οΏ½

𝑑𝑑𝑑𝑑

=πΏπΏπΈπΈπ‘…π‘…βˆ™π‘‘π‘‘ οΏ½1 βˆ’ π‘’π‘’βˆ’π‘…π‘…π‘‘π‘‘ 𝐿𝐿� οΏ½

𝑑𝑑𝑑𝑑=πΏπΏπΈπΈπ‘…π‘…οΏ½π‘…π‘…πΏπΏβˆ™ π‘’π‘’βˆ’π‘…π‘…π‘‘π‘‘ 𝐿𝐿� οΏ½ = πΈπΈπ‘’π‘’βˆ’π‘…π‘…π‘‘π‘‘ 𝐿𝐿�

Putting in the values for E, R, t and L gives…

𝒗𝒗𝑳𝑳 = πŸπŸπŸŽπŸŽπ’”π’”οΏ½βˆ’πŸπŸπŸŽπŸŽ

πŸ”πŸ”Γ—πŸŽπŸŽ.πŸπŸΓ—πŸπŸπŸŽπŸŽβˆ’πŸ”πŸ”πŸŽπŸŽ.𝟏𝟏 οΏ½

= πŸ”πŸ”.πŸŽπŸŽπŸ”πŸ”πŸ“πŸ“ 𝒗𝒗𝒔𝒔𝒔𝒔𝒍𝒍𝒔𝒔

Sample


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