Question

cmr , (a²+b²)(c²+d²)=(ac+bd)²-(ad-bc)²

Answer 1

VP=(a^2)(c^2)+2abcd+(b^2)(d^2)+ 
+(a^2)(d^2)-2abcd+(b^2)(c^2)
=a^2(c^2+d^2)+b^2(d^2+c^2)
=(a^2+b^2)(c^2+d^2)=VT