Chứng minh: 1) ( x + 1 )^2 - ( x - 1 )^2 = 4x. 2) ( x + y )^2 + ( x - y )^2 = 2x^2 + 2y^2. 3) ( x - 3 )^2 + 12xy = ( x - y )^2. 4) x^2 + y^2 = ( x + y )^2 - 2xy. 5) ( b + 1 )^2 - 2b( b + 1 ) + b^2 = 1