x^2/4+y^2/b^2=1 (b>4)
x=2cosu,y=bsinu
x^2+2y=(4cosu)^2+2bsinu
=-16(sinu)^2+2bsinu+16
=-(4sinu-b/4)^2+16+b^2/16
如果b16时,sinu=1时,x^2+2y最大=2
x^2/4+y^2/b^2=1 (b>4)
x=2cosu,y=bsinu
x^2+2y=(4cosu)^2+2bsinu
=-16(sinu)^2+2bsinu+16
=-(4sinu-b/4)^2+16+b^2/16
如果b16时,sinu=1时,x^2+2y最大=2