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${\displaystyle \frac{5}{3}}$2
$$\begin{array}{ccc}{(a+b)}^2& =& c^2+4\cdot \left(\frac{1}{2}ab\right)\\ a^2+2ab+b^2& =& c^2+2ab\\ a^2+b^2& =& c^2\end{array}$$