### Home > PC > Chapter 7 > Lesson 7.3.2 > Problem7-108

7-108.

$S=\frac{n(a_{1}+a_{n})}{2}$

$200=\frac{4(5+a_{n})}{2}$

400 = 20 + 40an

380 = 40an

95 = an

$\frac{95-5}{3}=30$

Series = 5 + 35 + 65 + 95

$S=\frac{a(r^{n}-1)}{r-1}$

$200=\frac{5(r^{4}-1)}{r-1}$

$200=\frac{5(r-1)(r+1)(r^{2}+1)}{r-1}$

200 = 5(r + 1)(r2 + 1)

By inspection = 3.

Series = 5 + 15 + 45 + 135