1)x^2+y^2
=(X+Y) ²-2XY
=5² -2*3
=25-6
=19
(2)(x-y)^2
=X²+Y²-2XY
=19-2*3(由上题得)
=19-6
=13
要祥裤熟谨拍简悉完全平方式的变贺滑形
1. x^2+y^2=(x+y)^2-2xy=19
2.(x-y)^2==(x+y)^2-4xy=13
(1)x^2+y^2 =(x+y)^2 -2xy=25-2×3=19
(2)(x-y)^2 =(x+y)^2 -4xy=25-4×3=13
(1)御团x^2+y^2 =(x+y)²-余含2xy=25-6=19
(竖拆笑2)(x-y)^2==(x+y)²-4xy=25-12=13