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x=f'(t)
dx/dt =f''(t)
y=tf'(t) -f(t)
dy/dt = tf''(t) +f'(t) -f'(t) = tf''(t)
dy/dx = (dy/dt)/(dx/dt) =t
d^2y/dx^2
=d/dx (dy/dx)
=d/dt (dy/dx) /(dx/dt)
=d/dt (dy/dx) / f''(t)
=d/dt (t) / f''(t)
=1/ f''(t)
dx/dt =f''(t)
y=tf'(t) -f(t)
dy/dt = tf''(t) +f'(t) -f'(t) = tf''(t)
dy/dx = (dy/dt)/(dx/dt) =t
d^2y/dx^2
=d/dx (dy/dx)
=d/dt (dy/dx) /(dx/dt)
=d/dt (dy/dx) / f''(t)
=d/dt (t) / f''(t)
=1/ f''(t)
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