>>406
>1/2u^2=k/y+C
(y')^2=(dy/dx)^2=2(k/y+C)
y'0=0
C=-k/y0
y'<0 for x>0
y'=-√(2(k/y-k/y0))
0<y<y0 for x>0
y=y0cos^2θ (0≦θ<π/2)
dy/dθ=-2y0sinθcosθ
√(2(k/y-k/y0))=(√(2k/y0))tanθ
dθ/dx=(dy/dx)/(dy/dθ)=(√(2k/y0))tanθ/2y0sinθcosθ=(√(k/2y0^3))/cos^3θ
dθcos^3θ=dx√(k/2y0^3)
(3sinθ-sin^3θ)/3=(√(k/2y0^3))x (θ=0 for x=0)
3sinθ-sin^3θ=(3-sin^2θ)sinθ=(2+cos^2θ)sinθ
(2+y/y0)√(1-y/y0)/3=(√(k/2y0^3))x
(y+2y0)√(y0-y)=(3√(k/2))x
(y+2y0)^2(y0-y)=(9/2)kx^2