Hi. i am using my phone to answer you, so i cant write much. to solve the 1st, you should convert cosine into sine. then you have a quadratic equation.when solving that,you earn the four general solutions: x=k.pi-pi/12 , x=k.pi+7pi/12 , x=k.pi-pi/4 , x=k.pi+3pi/4. which just the 1st is true! to solve the others, you can use the linke in my signature... .