a^2+b^2 =a^2+b^2+2ab-2ab =(a+b)^2-2ab =4^2-2×2 =12 (a-b)^2 =a^2+b^2-2ab =a^2+b^2+2ab-4ab =(a+b)^2-4ab =4^2-4×2 =8
