Question

Find a_(1) if d = 2.5, a_(15) = 33.5, S_(15 )= 240

Answer 1

`a_(1)=-19`

Explanation:

It is sum of arithmetic progression again.

`$S_(15)=(15)/(2) [2a_(1)+(15-1)\cdot 2.5]$`

`$240=(15)/(2) [2a_(1)+35]$`

`$240=15a_(1)+535$`

`-285=15a_(1)`

`a_(1)=-19`

Answer 2

Value of d is 20.

I hope it will help you.

Explanation: