ρ^2=a^2*cos2θ 怎样求导?怎样化为参数或直角坐标方程?

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ρ^2=a^2*cos2θ 怎样求导?怎样化为参数或直角坐标方程?
ρ^2=a^2*cos2θ 怎样求导?怎样化为参数或直角坐标方程?

ρ^2=a^2*cos2θ 怎样求导?怎样化为参数或直角坐标方程?
设 x = ρ*cosθ,y = ρ*cosθ
则原方程 ρ^4 =a^2 * ρ^2 *cos2θ
可以化为直角坐标方程:
(x²+y²)² = a²(x² - y²)
或者参方程:
x = a *√cos2θ * cosθ ,y = a *√cos2θ * sinθ
x ‘(θ) = a * [ - 2sin2θ *cosθ / (2*√cos2θ) + √cos2θ * (-sinθ)] = a * (-sin3θ) / √cos2θ
y ‘(θ) = a * [ - 2sin2θ *sinθ / (2*√cos2θ) + √cos2θ * (cosθ)] = a * (cos3θ) / √cos2θ
dy/dx = y ‘(θ) / x ‘(θ) = cos3θ / (- sin3θ) = - cot(3θ)