Question

Given Sn=2n^2 - 5n, find

(a) first term
(b) 9th term
(c) sum of the terms from the 4th term to the 8th term

Answer 1

a) -3

b) 117

c) 100

Explanation:

(a) 2(1)²-5(1)

1²= 1

2×1= 2

2-5= -3

(b) 2(9)²-5(9)

9²= 81

81×2= 162

162-45= 117

(c) 2(4)²-5(4) +2(8)²-5(4)

4²=16

16×2= 32

32-20= 12

8²=64

64×2= 128

128-40= 88

88+12= 100

hope it helps!