Even and Odd Trigonometric Functions
Points `P_t` and `P_(-t)` of unit circle has same x-coordinate and same y-coordinates with opposite signs (see figure).
This means that `cos(-t)=cos(t)`, `sin(-t)=-sin(t)`, i.e. function `y=cos(x)` is even and function `y=sin(x)` is odd. From definition it follows that functions `y=tan(x)` and `y=cot(x)` are odd.