已知:x^2+xy=3,xy+y^2=-2.求(1)x^2-y^2 (2)x^2+4xy+3y^2 (3)2x^2-xy

x^2+xy=3 .(I)
xy+y^2=-2 .(II)
(1).x^2-y^2 = (I) - (II) = 5
(2).x^2+4xy+3y^2 = (I) + 3*(II) = 3 + 3*(-2) = -3
(3).2x^2-xy-3y^2 = 2*(I) - 3*(II) = 2*3 - 3*(-2) = 12