Question

please answer this. I have exam today
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Answer 1

Step-by-step explanation:

Given,

the sum of first 8th term=136

and,

the sum of first 15th term=465

let,

the first term of the A.P. is 'a' and the common difference is 'd'.

S8=136

8/2[2a+(8-1)d]=136

4(2a+7d)=136

2a+ 7d =136/4

2a+7d =34__________(1)

S15=465

15/2[2a+(15-1)d]=465

2a+14d =(465/15)×2

2a+14d =62__________(2)

Now(1)-(2),

2a+7d-2a-14d= 34-62

-7d=-28

d=4

from (1),

2a+ 7×4= 34

a=(34-28)/2

a=3

Now,

S25= 25/2[2×3+(25-1)4]

=25/2[6+24×4]

=25/2×102

=25×51

=1275

That's your answer. I hope it will help you.