x^2/a^2+y^2=1
e^2=1-b^2/a^2=1-1/a^2=3/4
a^2=4
故椭圆E方程是x^2/4+y^2=1.
(2)显然圆心C坐标是(2t,0),当与Y轴相切时,半径r=x=2t=MN/2.
x=2t代入椭圆方程得到y^2=1-t^2
故有MN=2根号(1-t^2)=4t
1-t^2=4t^2
t=根号5/5
(3)S(OMN)=1/2*2t*MN=t*2根号(1-t^2)=2根号(t^2-t^4)=2根号[-(t^2-1/2)^2+1/4]
故当t^2=1/2,即有t= 根号2/2时,S有最大值是2*根号1/4=1.