a2+b2 is equal to what ?
We can write (a+b)2=(a+b)(a+b)=a2+ab+ba+b2
=a2+ab+ab+b2(becauseab=ba)=a2+2ab+b2
⇒a2+b2=(a+b)2-2ab(aftertranposingthevalues)
Hence a2+b2=(a+b)2-2ab