What is t (cos (t)) = 0?

In radians:

To get 0, only one part of the multiplication needs to be zero for the whole thing to be zero. Thus we get 2 equations:

t=0, and cos (t) = 0

for the first equation, it's pretty self-explanatory and sp we get 0 as one solution.

For the second equation, cos (pi/2) and cos (3pi/2) both equal 0 so both pi/2 and 3pi/2 are answers. Since there is no bound to this problem, the actual answer is:

(2Z+1) pi/2 and 0

(2Z+1) means all odd integers