Question

In an A.P. a = -2, d = 4 and S_(n) = 160, find n.

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WallaceMaster · Tutor for 5 years

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t the Associated Press, take that into account. $a=-2$ and $d=4$Given that the sum of first $n$ terms in the series is $S_{n}=160$We know that the sum of first $n$ terms in the series is $S_{n}=\frac{n}{2}(2a+(n-1)d)$$\Rightarrow \frac{n}{2}(2a+(n-1)d)=160$$\Rightarrow \frac{n}{2}(-4+4(n-1))=160$$\Rightarrow n(n-2)=80$$\Rightarrow n=10,-8$Since $n$ is positive integer , the value of $n=10$

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