用点差法解决.设弦的两个端点是A（x1,y1),B(x2,y2)则x1^2/36+y1^2/9=1,x2^2/36+y2^2/9=1相减得x1^2-x2^2+4y1^2-4y2^2=0移项后因式分解可得(x1+x2)(x1-x2)=-4(y1+y2)(y1-y2)由于A,B中点为p(4,2),所以x1+x2=8,y1+y2=4...