The correct option is
B 7: 16
For 1st AP
Let first term be a common difference be d
sum of n terms sn=n2(2a+(n−1)d)
nth term an=a(n−1)d
Similarly for 2nd AP
Let first term =A common difference be D
Sn=n2(2A+(n−1)D) & nth term =An=A+(n−1)D
We need ratio of 12th term
i.e., a12offirstAPA12ofsecondAP
=a+(12−1)dA+(12−1)D
=a+11da+11D
It is given that
Sumofntermsof1stAPSumofntermsof2ndAP=3n+87n+15
∴2a+(n−1)d2A+(n−1)D=3n+87n+15 ………….(1)
2(a+(n−12)d)2(A+(n−12)D)=3n+87n+15 ………………(1)
we need to find a+11dA+11D
Hence n−12=11
n−1=22
n=23
Putting n=23 in (1)
a+(23−12)dA+(23−12)D=3×23+187×23+15
∴a+11dA+11D=716
Hence ratio of their 12th term is 716 i.e., 7:16.
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