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Question

If ninth term of an A.P. is 20, find the sum of its seventeen term.

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Solution

Let a be the first term and d be the common difference of the A.P.

Then according to the problem, a+8d=20......(1).

Sn=n2×(2a+(n1)d)

Now, the sum of 17 th term is =172×(2a+16d)=172×2×20=340. [ Using (1)].

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