直线y=mx与圆x^2+y^2+8x-6y+21=0交于点P,Q,求OP向量乘以OQ向量

x^2+y^2+8x-6y+21=0设p(x1,mx1) q(x2.mx2)向量OP点乘OQ=x1x2+m^2x1x2=(m^2+1)x1x2x1x2是方程x^2+m^2x^2+8x-6mx+21=0的两解x1x2=21/(m^2+1)向量OP点乘OQ=x1x2+m^2x1x2=(m^2+1)x1x2=21