y=mx代入方程X的平方+Y的平方+8X-6Y+21=0 x^2+m^2x^2+8x-6mx+21=0 (1+m^2)x^2+(8-6m)x+21=0 x1x2=21/(1+m^2) P(x1,mx1)Q(x2,mx2) |OP|=√(1+m^2)|x1| |OQ|=√(1+m^2)|x2| |OP||OQ|=(1+m^2)|x1x2|=21