x = a(cost)^3, y = a(sint)^3
dx/dt = -3asint(cost)^2, dy/dt = 3acost(sint)^2
y' = dy/dx = (dy/dt)/(dx/dt) = -tant
y'' = dy'/dx = (dy'/dt)/(dx/dt) = -(dtant/dt)/(dx/dt)
= -(sect)^2/[-3asint(cost)^2] = (sect)^4/(3asint)
曲率 K = |y''|/(1+y'^2)^(3/2) = |(sect)^4/(3asint)|/[1+(tant)^2]^(3/2)
= |sect/(3asint)| = 1/|3asintcost)| = 2/|3asin(2t)|
在 t = t0 处， K = 2/|3asin(2t0)|
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