A2+B2+C2+2AB+2BC+2AC=?
A2+B2+C2+2AB+2BC+2AC
=A×A+A×B+A×C+B×A+B×B+B×C+C×C+C×B+C×A
=A(A+B+C)+B(A+B+C)+C(A+B+C)
=(A+B+C)2