1. The problem statement, all variables and given/known data Let c=cos(2pi/5). It can be shown that (4c^2)+(2c)-1=0. Use this fact to prove that a 72degree angle is constructible. 2. Relevant equations 3. The attempt at a solution I can see that using the equation and what c equals that you get the statement 0=0 and I know that 2pi/5 relates to 72degree angle on the unit circle but I have no idea how to formally prove it. Also can equate cos(2pi/5) but don't see how that would help either.