f（x,y）=x²+2xy-y²+6y-4
=x^2+2xy+y^2-2y^2+6y-9/2+1/2
=(x+y)^2-2(y-3/2)^2+1/2
f(x,y)min=1/2
此时y=3/2x=-3/2
即当=-3/2,y=3/2时,函数有最小值f(x,y)min=1/2