(x+y)^2=x^2+y^2+2xy=9 (1)
(x-y)^2=x^2+y^2-2xy=4 (2)
(1)-(2)得：4xy=5,2xy=5/2
所以：x^2+y^2=9-5/2=13/2
(1)+(2)得：2x^2+2y^2=13,x^2+y^2=13/2
x+y=±3
x-y=±2
A+B
=-3x^2+2y^2-2x^2+3y^2
= -5x^2+5y^2
= 5(y+x)(y-x)
=±5*3*2
=±30
A-B
=-3x^2+2y^2-(-2x^2+3y^2)
=-3x^2+2y^2+2x^2-3y^2
= -( x^2+y^2)
= -13/2看不懂请问哪一步看不明白？第一步，谢谢(x+y)^2=x^2+y^2+2xy ---完全平方公式(x+y)^2=9 ----已知条件所以 x^2+y^2+2xy =9