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Gee, I wish I could use my TI – 83!
For each of the following sequences, determine the common difference and the level at which it occurs.
1. -3, 0, 5, 12, 21
2. 1, -2, -9, -20, -35
3. 6, 10, 14, 18,
4. - 10, -27, -56, -97
3 5 7 9
2 2 2
d = 2 at Level D2 Quadratic
-3 -7 -11 -15
-4 -4 -4 d = -4 at Level D2 Quadratic
4 4 4d = 4 at Level D1 Arithmetic
-17 -29 -41
-12 -12d = -12 at Level D2 Quadratic
Now use the function to generate the first four terms for each of these quadratic functions. Determine the common difference. What relation does it have with the coefficient of the a2 term?
132 xxy 124 2 xxy
xxy 26 22xy
Quadzilla
X Y
1 5
2 11
3 19
4 29
X Y
1 -5
2 -19
3 -41
4 -71
X Y
1 7
2 26
3 57
4 100
X Y
1 -2
2 -8
3 -18
4 -32
d is 2 at D2
a is 1d is -8 at D2
a is -4
d is 12 at D2
a is 6d is -4 at D2
a is -2
Is there a PATTERN here?
In a Quadratic Sequence there is a special relationship between a & d!
The Relationship between a & d in a Quadratic Sequence
The difference “d” from Level D2 is twice the coefficient of the n 2 or x 2 term in the general formula.
2
2
da
or
ad
So to determine the formula or rule for the nth term of a certain quadratic sequence, we must first find the common difference
and divide by 2 to find the coefficient “a”!
cbnantn 2
The general formula for the nth term of a quadratic sequence is
Let’s determine the first 5 terms
cbacbat )1()1( 21
cbacbat 24)2()2( 22
cbacbat 39)3()3( 23
cbacbat 416)4()4( 24
cbacbat 525)5()5( 25
cbacbacbacbacba 525,416,39,24,
To determine the common difference, we subtract backwards. The first five terms of this sequence are”
babababa 9753
aaa 222
22 DDifferenceCommonthea
13 DinTermFirsttheba
SequencetheinTermFirstthecba
Now You Try! Use “d” to determine “a” and then solve two equations!
The sequence is 7, 16, 31, 52, 79
9 15 21 27
6 6 6d2 =
7 cba
93 ba
62 a 3a9)3(3 b99 b0b
703 c4c
43, 2 nissequencequadraticthisforequationtheSo
The Formula for the nth term of a Quadratic Sequence is
cbnantn 2
1. To algebraically determine the formula or expression for the nth term of a Quadratic Sequence we need to know the formula.
2. We need to know the common difference in order to determine the coefficient “a”.
7, 16, 29, 46, 67
3. We need to use the information from two terms to set up two equations.
7&171 ntnusgivest
16&2162 ntnusgivest
1
2
4. We need to solve the resulting System of Equations to determine “b” & “c”
5. We need to replace a, b, & c in the general formula.
STEPS
The Sequence 7, 16, 29, 46, 67
1 cbnantn 2
2 d = 4 so a = 2
3 Two terms to set up two equations.
cb
cb
cb
cbnant
tnt
n
n
5
27
)1()1(27
7&17
2
2
1
cb
cb
cb
cbnant
tnt
n
n
28
2816
)2()2(216
16&216
2
2
2
4 Solve for b & c by solving the System of Equations.
cb5cb28SUBTRACT
b3cb5
SUBTITUTE to find the other variable.
c 35c2
METHOD
232 2 nntn
Now we have
Page 13 # 40, 41, 42
16 # 5, 8, 9
Remember, Homework is not meant to
be a burden. It is meant to help you to
reinforce the lesson and it helps you to
remember the steps and
proves whether you
understand!