Question

求值（x^2+xy)/(x^2-2xy-3y^2)*(3y-x)/(x-y)+(x^-1+y^-1)÷(x^-1-y^-

Answer 1

（x^2+xy)/(x^2-2xy-3y^2)*(3y-x)/(x-y)+(x^-1+y^-1)÷(x^-1-y^-1),

=x(x+y)/(x-3y)(x+y)×(3y-x)/(x-y)+(x+y)/(y-x)

=-x/(x-y)+(x+y)/(y-x)

=(-x-x-y)/(x-y)

=(-2x-y)/(x-y)

当x=2,y=-1时

原式=(-4+1)/(2+1)=-1