values of trig functions
135° is in Quadrant II, and has a reference angle of 45°. (180° - 135° = 45°) Since tangent is negative in Q II, tan 135° = - tan 45°. Now do you see?
315° is in Quadrant IV, and has a reference angle of 45° also. (360° - 315° = 45°) Since cosine is positive in Q IV, cos 315° = cos 45°. OK?