the sum of first 9th term of an ap is 81 and that of 8th first 21 term is 400 find first and common difference
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Step-by-step explanation:
S
9
=81 =>
2
9
[2a+(9−1)d]=81
S
20
=400 =>
2
20
[2a+(20−1)d]=400
=>18a+72d=162
=>20a+190d=400
=>2a+19d=40
18a+72d=162
18a+171d=360
______________
−99d=−198
d=2.
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