L:y=k(x-1)
x^2/4+y^2/3=1
3x^2+4y^2=12
3x^2+4*[k(x-1)]^2=12
(3+4k^2)x^2-8k^2*x+4k^2-12=0
yM+yN=8k^2/(3+4k^2),yM*yN=(4k^2-12)/(3+4k^2)
(yM-yN)^2=(yM+yN)^2-4yM*yN=
(y1-y2)^2=(y1+y2)^2-4y1*y2=
|y1-y2|=√[(y1+y2)^2-4y1*y2]
S△MON
=S△MOD+S△NOD
＝(1/2)底*|OD|*高|yM| +(1/2)底*|OD|*高|yN|
=(1/2)*|OD|*|yM|+(1/2)*|OD|*|yN|
=(1/2)*|OD|*(|yM|+|yN|)
=(1/2)*1*|yM-yN|
=(1/2)*|yM-yN|
=(1/2)*|y1-y2|
=(1/2)*√[(y1+y2)^2-4y1*y2]