Question

14. The nth term of a quadratic sequence is n²+ 3n-2

(a) Find the fourth term of this sequence.
Here are the first five terms of a different quadratic sequence.
1, 7, 17, 31, 49
(b) Find, in terms of n, an expression for the nth term of this sequence.

Answer 1

Explanation:

14a)

the 4th term = 4² + 3×4 - 2 = 16 + 12 - 2 = 26

b)

1, 7, 17, 31, 49

s1 = 1

s2 = s1 + 6

s3 = s2 + 10

s4 = s3 + 14

s5 = s4 + 18

the added term d grows with every step by 4. starting with a constant "+ 2" on top of -1 to create s1.

d1 = 2

d2 = d1 + 4

d3 = d2 + 4

dn = dn-1 + 4 = d1 + 4(n-1)

s1 = -1 + d1

s2 = s1 + d2 = -1 + d1 + d1 + 4

s3 = s2 + d3 = -1 + d1 + d1 + 4 + d1 + 4 + 4

s4 = s3 + d4 = -1 + d1 + d1 + 4 + d1 + 4 + 4 + d1 + 4 + 4 + 4

sn = -1 + n×d1 + sum(4(n - 1)) = -1 + 2n + 4n(n - 1)/2 =

= -1 + 2n + 2n(n - 1) = -1 + 2n(1 + n - 1) = -1 + 2n² =

= 2n² - 1