a) (a+b)^2=(a-b)^2 +4ab
=(a+b)^2=....................................
=(.........-2ab+...........) +4ab
=(a-b)^2 + ...............
b) (a-b)^2 = (a+b)^2 - 4ab
=(a-b)^2=....................................
=(...............................+b^2) -4ab
= ( a+b)^2 - 4ab