## Calculus with Applications (10th Edition)

We have a definition of periodic: A function $y=f(x)$ is periodic if there exists a positive real number $a$ such that $f(x)=f(x+a)$ for all values of x in the domain of the function. $\sin x=\sin(\pi+x)$, $\tan2x=\tan(2x+\pi)$. Hence, all six of the basic trigonometric functions are periodic.