Bilinear form/Symmetric/Polarization formula/2 terms/Exercise

Let ${}K$ be a field, and suppose that its characteristic is not ${}2$. Let ${}\left\langle -,-\right\rangle$ denote a symmetric bilinear form on a ${}K$-vector space ${}V$. Show

${}\left\langle v,w\right\rangle ={\frac {1}{4}}{\left(\left\langle v+w,v+w\right\rangle -\left\langle v-w,v-w\right\rangle \right)}\,.$

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