Question

若x2+4x+y2-6y+13=0,则(x+2y)(x-2y)的值

Answer 1

x^2+4x+y^2-6y+13=0 (x^2+4x+4)+(y^2-6y+9)=0 (x+2)^2+(y-3)^2=0 x=-2,y=3 (x+2y)(x-2y)=x^2-4y^2=4-4*9=-32