Sai đề,sửa đề : \(a-b=c+d\)
\(a-b=c+d\)
\(\Rightarrow a=b+c+d\)
\(\Rightarrow a^2+b^2+c^2+d^2=\left(b+c+d\right)^2+b^2+c^2+d^2\)
\(\Rightarrow b^2+c^2+d^2+2bc+2bd+2cd+b^2+c^2+d^2\)
\(=\left(b+c\right)^2+\left(c+d\right)^2+\left(b+d\right)^2\left(đpcm\right)\)