Calculus Parametric Curve

Question

For what value(s) of t, with 0 ≤ t ≤ 4, is the slope of the curve undefined for the graph defined by x = 2t - 2 sin(t), y = 2 - 2 cos(t) ?

Answer 1

The answer is whenever cos t = 1.

Why?

dy/dx = (dy/dt)/(dx/dt)

What would make dy/dx undefined?

When t is an integer multiple of 2pi. what is the value of dy/dx?