设弦AB的坐标分别是:A(X1,Y1),B(X2,Y2)
中点M(X,Y)
则有:X1+X2=2X,Y1+Y2=2Y.
(Y1-Y2)/(X1-X2)=3.
A,B坐标代入得:
X1^2/16+Y1^2/4=1
X2^2/16+Y2^2/4=1
二式相减得:
(X1+X2)(X1-X2)/16+(Y1+Y2)(Y1-Y2)/4=0
2X/(X1-X2)/16=-2Y(Y1-Y2)/4
X=4Y*(Y1-Y2)/(X1-X2)
所以,X=4Y*3=12Y