Problem: Quadratic identities

Problem

Prove that $\left( a+b \right)\left( c+d \right)=ac+ad+bc+bd$ using the distributive law $a\left( b+c \right)=ab+ac$ . Hint: let $e=a+b$ . Then $\left( a+b \right)\left( c+d \right)=e(c+d)$ . Now use the distributive law.. several times.