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  4. A student read common difference of an A. P. as -2 instead of 2 and got the sum of first 5 terms as -5. Actual sum of first five terms is

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Q. A student read common difference of an A. P. as $-2$ instead of $2$ and got the sum of first $5$ terms as $-5$. Actual sum of first five terms is

BITSATBITSAT 2020

Solution:

$\frac{5}{2}\{2 a+4(-2)\}=-5 \Rightarrow 2 a=6$
$\therefore $ Actual sum $=\frac{5}{2}\{6+4(2)\}=35$