A（x1，y1) B(x2,y2）过A的切线y=4x1(x-x1)+y1 B的切线y=4x2(x-x2)+y2交点M（(x1+x2)/2,2x1x2)设过P点的直线为y=k（x-1)+4联立2x²-kx+k-4=0则M（k/4,k-4)所以y=4x-4
