a) A(ABCD)= (a+b)^2= a^2+2ab+b^2
b) A(MRQD)+A(PRNB)=a^2+b^2
c) => A(ABCD) > A(MRQD)+A(PRNB)
=> a^2+2ab+b^2 > a^2+b^2 cu 2ab