If the sum of n terms of an A.P. is 5n + 2n, find the A.P. and its 15 term.
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Answer:
5n+2n = AP
when n = 1
5(1) + 2(1) = 7 = a1
5(2) + 2(2) = 14 = S2
∴ a2 = S2 - S1
a2 = 14 - 7 = 7
7, 14, …
a= 7
d=7
a15 = a+14(d)
a15= 7+14(7)
a15= 7+98 = 105
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