Presumably you mean the second derivative of y with respect to x, d²y/dx².
Compute the first derivative. By the chain rule,

Differentiate the two parametric equations with respect to t :


Then the first derivative is

Now, dy/dx is a function of t, so we can denote it by, say, dy/dx = f(t). Then by the chain rule, the second derivative will be

Differentiating f(t) :

Then the second derivative is

and since y = cos(t), we can go on to say
