We have the functions
x=t-sintx=t−sint and y=1-costy=1−cost
If we find out dy/dtdydt and dx/dtdxdt we can divide them and find dy/dxdydx
(dy/dt)/(dx/dt)dydtdxdt ->→ (dy/canceldt)/(dx/canceldt) -> dy/dx
color(red)1. y=1-cost
Differentiate both sides with respect to t
d/dty=d/dt(1-cost)
dy/dt=0-(-sint)
dy/dt=sint -> color(red)A
color(red)2. x=t-sint
Differentiate both sides with respect to t
d/dtx=d/dt(t-sint)
dx/dt=1-cost -> color(red)B
Now we divide color(red)A and color(red)B
(dy/dt)/(dx/dt)=sint/(1-cost)
dy/dx=sint/(1-cost)