1. The problem statement, all variables and given/known data 18cos^2x+3cosx-1=0 it's a quadratic equation whose solutions are x= 1.4033+-2npi and x=1.9106+-2npi 2. Relevant equations 3. The attempt at a solution The answers to this problem are given as x= +-1.4033+-2npi and x=+-1.9106+-2npi Why are x's both positive and negative? Thanks.