x=cosØ
x^2=cosØ)^2
y=cos2Ø+1
cos2Ø=y-1
2(cosØ)^2-1=y-1
(cosØ)^2=y/2
即
x^2=y/2
y=2x^2(抛物线)
x=t+(1/t)
y=t-(1/t)(t为参数)
两式都平方得
x^2=t^2+2+1/t^2,
y^2=t^2-2+1/t^2
x^2-y^2=4 (双曲线)
x=5cosA
y=2sinA
cosA=x/5
(cosA)^2=x^2/25
sinA=y/2
(sinA)^2=y^2/4
(cosA)^2+(sinA)^2=x^2/25+y^2/4
x^2/25+y^2/4=1(椭圆)