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

Let $K$ be a field, and suppose that its characteristic is not $2$. Let $⟨-,-⟩$ denote a symmetric bilinear form on a $K$-vector space $V$. Show

$⟨v,w⟩=\frac{1}{4}(⟨v+w,v+w⟩-⟨v-w,v-w⟩).$

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