Pls help to solve these maths?

Question

the 8th term of a GP is -7/32, 
find its common ratio if its first term is 28. 

No(3) an AP has 15 terms and common difference of -3,find its first and last terms, 
if its sum is -120.

Answer 1

8th term of a GP is -7/32, 

ar^7=-7/32

28r^7=-7/32

r^7=-1/128

r=-1/2

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Sum of the n-term in AP is  S_n = (n/2)[2a +(n -1)d]

Here n=15,Sn =-120ans d=-3

-120= (15/2)[2a +(15 -1)(-3)]

-16=2a +(15 -1)(-3)

-16=2a -42

a=13

First term is 13

last term,a+(n-1)d

t15=13+(15-1)(-3)

    =-29