设弦AB,A（x1,y1),B(x2,y2),
x1^2/36+y1^2/9=1,(1)
x2^2/36+y2^2/9=1,(2)
(1)-(2)式,
(x1^2-x2^2)/36+(y1^2-y2^2)/9=0,
1/4+[(y1-y2)/(x1-x2)]*{[(y1+y2)/2]/[(x1+x2)/2]},(3)
其中弦的斜率k=(y1-y2)/(x1-x2),
(y1+y2)/2=2,
(x1+x2)/2=4,
代入（3）式,
1/4+k*2/4=0,
k=-1/2,
∴弦所在直线方程为：(y-2)=-(x-4)/2,
即：y=-x/2+4.