Finite Differences

The Key To Unlocking The Pattern

Patterns may be represented by various rules.

Linear—First Differences—an+b

Quadratic—Second Differences—an2+bn + c

Cubic—Third Differences—an 3+bn2+cn + d

Quartic—Fourth Differences—an 4 +bn 3+cn2+dn + e

This pattern continues but we will not generally study any patterns higher than fourth differences.

Given a Pattern0, 16, 64, 162, 328

• STEP 1:Create a table and fill in the values of the given pattern. “n” represents the number of the term (i.e. first term, second term, etc). There will not be a “zero” term at this time so create a spaceholder for that term.

Step 1: Create the table

n01 02 163 644 1625 328

Given a Pattern0, 16, 64, 162, 328

• STEP 2:Calculate the difference from term 1 to term 2, term 2 to term 3, etc.

These are called the first differences.

As you work down the table, you add. +As you work up a the table you subtract. -

Step 2: Calculate 1st differences

n01 02 163 644 1625 328

16

48

98

166

Are the first differences equal?

If yes, then you have a linear pattern.

If no, then continue to find the second differences.

Given a Pattern0, 16, 64, 162, 328

• STEP 3:If the first differences are not equal, then create a spaceholder at the top of this column.

Then calculate the difference from 16 to 48; from 48 to 98, etc.

These are called the second differences.

As you work down the table, you add. +As you work up a the table you subtract. -

Step 3: Calculate 2nd Differences

n01 02 163 644 1625 328

16

48

98

166

Are the second differences equal?

If yes, then you have a quadratic pattern.

If no, then continue to find the third differences.

32

50

68

Given a Pattern0, 16, 64, 162, 328

• STEP 4:If the second differences are not equal, then create a spaceholder at the top of this column.

Then calculate the difference from 32 to 50 and from 50 to 68.

These are called the third differences.

As you work down the table, you add. +As you work up a the table you subtract. -

Step 4: Calculate 3rd Differences

n01 02 163 644 1625 328

16

48

98

166

Are the third differences equal?

If yes, then you have a cubic pattern.

If no, then continue to find the fourth differences.

32

50

68

18

18

Given a Pattern0, 16, 64, 162, 328

• STEP 5:Since the third differences are the same in this case, we know we have a cubic pattern.

We now need to work backwards and fill in our shapes that are serving as placeholders.

As you work down the table, you add. +As you work up a the table you subtract. -

Step 5: Calculate 3rd Differences

n01 02 163 644 1625 328

16

48

98

166

32

50

68

18

18

18We know the third differences are all the same (18) so we may fill in this difference to be 18.

Step 6: Work backwards to fill in the placeholders.

n01 02 163 644 1625 328

16

48

98

166

32

50

68

18

18

18We know the third differences are all the same (18) so we may fill in this difference to be 18.

Now we need to fill in the . Ask yourself 32 – 18 = what?

Step 6: Work backwards to fill in the placeholders.

n01 02 163 644 1625 328

16

48

98

166

32

50

68

18

18

18We find that the = 14. 14

Now we need to fill in the . Ask yourself 16 – 14 = what?

Step 6: Work backwards to fill in the placeholders.

n01 02 163 644 1625 328

16

48

98

166

32

50

68

18

18

18We find that the = 2. 14

Now we need to fill in the . Ask yourself 0 – 2 = what?

2

Step 6: Work backwards to fill in the placeholders.

n01 02 163 644 1625 328

16

48

98

166

32

50

68

18

18

18We find that the = - 2. 14

2- 2

Given a Pattern0, 16, 64, 162, 328

• PART BNow that we have completed the table, we need to look at the corresponding table that fits a third finite difference pattern. Every third difference can be modeled by the rule: an 3+bn2+cn + d

As you work down the table, you add. +As you work up a the table you subtract. -

Step 1: Create the table by substituting the value of n.

n an 3+bn2+cn + d01 a(1)3+b(1)2+c(1) + d2 a(2)3+b(2)2+c(2) + d3 a(3)3+b(3)2+c(3) + d4 a(4)3+b(4)2+c(4) + d5 a(5)3+b(5)2+c(5) + d

This is the simplified table. Now we will calculate the first differences just like we did in the last table.

n an 3+bn2+cn + d01 a + b + c + d2 8a+4b+2c + d3 27a+9b+3c + d4 64a+16b+4c+d5 125a+25b+5c+d

7a +3b+c

19a +5b+c

37a +7b+c

61a +9b+c

This is the simplified table. Now we will calculate the second differences just like we did in the last table.

n an 3+bn2+cn + d01 a + b + c + d2 8a+4b+2c + d3 27a+9b+3c + d4 64a+16b+4c+d5 125a+25b+5c+d

7a +3b+c

19a +5b+c

37a +7b+c

61a +9b+c

12a+2b

18a+2b

24a+2b

This is the simplified table. Now we will calculate the third differences just like we did in the last table.

n an 3+bn2+cn + d01 a + b + c + d2 8a+4b+2c + d3 27a+9b+3c + d4 64a+16b+4c+d5 125a+25b+5c+d

7a +3b+c

19a +5b+c

37a +7b+c

61a +9b+c

12a+2b

18a+2b

24a+2b

6a

6a

We know the third differences are all the same (6a) so we may fill in this difference to be 6a.

6a

Now we will work backwards to fill in the shapes.

n an 3+bn2+cn + d01 a + b + c + d2 8a+4b+2c + d3 27a+9b+3c + d4 64a+16b+4c+d5 125a+25b+5c+d

7a +3b+c

19a +5b+c

37a +7b+c

61a +9b+c

12a+2b

18a+2b

24a+2b

6a

6a

6a

6a+2b

Now we will work backwards to fill in the shapes.

n an 3+bn2+cn + d01 a + b + c + d2 8a+4b+2c + d3 27a+9b+3c + d4 64a+16b+4c+d5 125a+25b+5c+d

7a +3b+c

19a +5b+c

37a +7b+c

61a +9b+c

12a+2b

18a+2b

24a+2b

6a

6a

6a

6a+2ba+b+c

Now we will work backwards to fill in the shapes.

n an 3+bn2+cn + d01 a + b + c + d2 8a+4b+2c + d3 27a+9b+3c + d4 64a+16b+4c+d5 125a+25b+5c+d

7a +3b+c

19a +5b+c

37a +7b+c

61a +9b+c

12a+2b

18a+2b

24a+2b

6a

6a

6a

6a+2ba+b+cd

Now we will compare the values in the shapes for the two tables.

Table for the Given Pattern

Table for 3rd Differences

- 2

Conclusion

d = -2

6a1818 = 6aa= 3

6a+2b

d

14 6a+2b = 146(3)+2b=14b= -2

a+b+c2 a+b+c= 23+ -2 +c = 21+c=2c=1

Now that we know the values of a, b, c, and d, we can write the rule.

an 3+bn2+cn + d

Rule =3n3+ (-2) n2+1n + (-2)Rule = 3n3-2n2+n -2

We can now use this rule to calculate any specific term. For instance, if we want to know the 11th term, we substitute n = 11.

Term 11= 3(11)3-2(11)2+ 11 -2Term 11 = 3760

This same procedure will be followed for any difference.

For first differences, you stop whenever the first differences are =. You will be finding a and b.

For second differences, you stop whenever the second differences are =. You will be finding a, b, and c.