右焦点F(1,0)
设弦AB是A(X1,Y1)B(X2,Y2)中点P是(X,Y)
有X1+X2=2X,Y1+Y2=2Y
又x1^2/5+y1^2/4=1
x2^2/5+y2^2/4=1
相减得(x1+x2)(x1-x2)/5+(y1+y2)(y1-y2)/4=0
即AB斜率k=(y1-y2)/(x1-x2)