Double Angle Extra Practice Help

Hi Mr. G, I didn’t understand 10b or c, since multiplying cos 112.5 degrees results in a relative obtuse angle of 135 degrees, but from the 135 you can get a RAA if 45 degrees (but I’m not sure if that’s allowed). Also, did c) I think we have to use the quadratic formula, but I don’t know what values to sub in