Question

Find the specified value:
S5=31/4 , r=1/2 , a1=?
Please explain how to solve this.

Answer 1

is it the GP ?

Answer 2

if yes, use the formula :
Sn = a(1 - r^n)/(1-r)

Answer 3

so,
S5 = 31/4
a(1 - r^n)/(1-r) = 31/4
a(1 - (1/2)^5)/(1- 1/2) = 31/4
a(1- 1/32)/(1/2) = 31/4
a(31/32) * 2 = 31/4
a (31/16) = 31/4
a = (31/4)/(31/16)
a = ...