It is rather obvious that we will have to deal with a trigonometric function. Let us see... the first hint is that a function is not defined somewhere. cos and sin are defined for all real numbers... what about tan though? Well, as we approach pi/2, tan becomes larger and larger and it is not defined at pi/2. Given that the tan function has a period of pi (cos and sin have a period of 2pi), this function is the right candidate. Since it is periodic and it is not defined at pi/2, it cannot be defined at any x=pi/2+-npi (if it was defined for some n, then it would have the same value at pi/2 due to periodicity). Hence, a function that works is tanx, or 2*tanx or tanx+6.
