In an AP, given an=4, d=2, sn=-14, Find n and a.

Ans. n=7 and a=-8 ....This is from the chapter AP.
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AP=a+a+d+a+2d+a+3d+...+a+nd=a+a+2+a+4+a+6+...+a+2n. a+2n=4, a=4-2n. 

S[n]= a+na+d(n(n+1)/2)=-14; 4-2n+4n-2n^2+n^2+n=-14; n^2-3n-18=0=(n-6)(n+3)=0. So n=6, a=4-12=-8.

Counting from 0, n=6 and a=-8=a[0]; counting from 1, n=7 and a=-8=a[1].

by Top Rated User (1.2m points)

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