Which of the following are solutions to the equation 2cos^2(x) - 1 = 0? check all that apply. a. 3pi/4 b. 15pi/4 c. pi/8 d. -7/pi/4

The equation 2cos^2(x) - 1 = 0 we manupulate by adding 1 to both sides and then dividing both sides by 2.                                  
                                 2cos^2(x) - 1 = 0
                                       2cos^2(x) = 1
                                         cos^2(x) = 1/2 
                                             cos(x) = ±√1/2  
                                             cos(x) = ±√(2)/2        
The answers to your question are A. 3pi/4, B. 15pi/4, and D. -7pi/4 as they all are angles with a reference angle of pi/4. They will have a cosine ratio of √(2)/2 or -√(2)/2