cos75°*cos15°-sin225°*sin15°= cos75°*cos15°- sin(180°+ 45°)*sin15°= cos75°*cos15°+ sin45°*sin15°= (√6-√2)/4 × (√6+√2)/4 + √2/2 × (√6-√2)/4= (6-2)/16 + √2/2 × (√6-√2)/4= 1/4 +...