a2+b2 (a-b)2
=a2+2ab+b2-2ab =a2-2ab+b2
=(a+b)2-2ab =a2+2ab+b2-4ab
=42-5/2×2 =(a+b)2-4ab
=16-5 = 42-5/2×4
=11 =16-10
=6