点P(2,1)是椭圆x2/9+y2/4=1内一点,则以P为中点的弦所在直线的方程为

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设过P点弦为AB,A（x1,y1),B(x2,y2),
x1^2/9+y1^2=1,(1)
x2^2/9+y2^2=1,(2)
(1)-（2）式,
(x1^2-x2^2)/9+(y1^2-y2^2)/4=0,
4/9+[(y1-y2)/(x1-x2)]*{[(y1+y2)/2/[(x1+x2)/2]}=0,(3)
弦直线方程斜率k=(y1-y2)/(x1-x2),
(y1+y2)/2=1,(x1+x2)/2=2,
代入（3）式,
4/9+k*1/2=0,
k=-8/9,
直线方程为：(y-1)=-8(x-2)/9,
即8x+9y-25=0.