PROBLEM is 1-Dimensional wave equation, the solution should be periodic in nature. If K is a POSITIVE number, then the solution comes out to be (c7 epx c+e-px cc8)(c7 ept+e-ptc8) and if k is positive the solution comes out to be (ccos(px/c) + c SIN(px/c))(c cospt + c sinpt). Now, since it should be periodic, the solution is (ccos(px/c) + c sin(px/c))(c cospt + c sinpt).