a^2+b^2=（a+b）^2-2ab=5^2-2*2=21(a-b)^2=a^2+b^2-2ab=21-2*2=17
a^2+b^2=a²+2ab+b²-2ab=(a+b)²-2ab=2²-2*5=4-10=-6(a-b)²=a²-2ab+b²=a²+b²-2ab=-6-2*5=-16
a^2+b^2=（a+b）^2-2ab=21(a-b)^2=（a+b）^2-4ab=17
