联立 x^2=-4/k
所以x1x2=4/k x1+x2=0
2x1y2-7x2y1=2x1*kx2-7x2*kx1=(-5k)*x1x2=(-5k)*(4/k)=-20
k(x1^2+x2^2)=k[(x1+x2)-2x1x2]=-2k*x1x2=-2k*(4/k)=-8