IIT-JEE 1982Question 366
$A_1,A_2,\ldots,A_n$ are the vertices of a regular plane polygon with $n$ sides and $O$ is its centre. Show that $$\sum_{i=1}^{n-1}\left(\overrightarrow{OA_i}\times\overrightarrow{OA_{i+1}}\right)=(1-n)\left(\overrightarrow{OA_2}\times\overrightarrow{OA_1}\right)$$
Vector Algebra
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