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Question

The sum of n terms of an A.P is 3n2+5n. Find the A.P hence, find its 15th term.

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Solution

n2[2a+(n1)d]=3n2+5n
n2(2a+(n1)d]=n(3n+5)
2a+(n1)d=6n+10
d=6n+10=2a(n1)
15th term
a+(n1)d=a+14d
=a+14×(6×15+102a14)
15th term =a+90+102a=100a.

1212446_1300213_ans_e3932dee1e65402aa8deb4d31efa566f.JPG

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