Ta có: \(a^2+b^2=\left(a+b\right)^2\)
\(\Leftrightarrow a^2+b^2-\left(a^2+2ab+b^2\right)=0\)
\(\Leftrightarrow a^2+b^2-a^2-2ab-b^2=0\)
hay -2ab=0
\(\Leftrightarrow ab=0\)
\(\Leftrightarrow\left\{{}\begin{matrix}a=0\\b=0\end{matrix}\right.\Leftrightarrow a=b\)(đpcm)