−cos2a−cos2a+cos4a=tg4a(1−sin2a)−sin2a(1−sin2a)(1−cos2a)−cos2a(1−cos2a)=tg4a(1−sin2a)(1−sin2a)(1−cos2a)(1−cos2a)=tg4a(1−sin2a)2(1−cos2a)2=tg4acos4asin4a=tg4a
−cos2a−cos2a+cos4a=tg4a(1−sin2a)−sin2a(1−sin2a)(1−cos2a)−cos2a(1−cos2a)=tg4a(1−sin2a)(1−sin2a)(1−cos2a)(1−cos2a)=tg4a(1−sin2a)2(1−cos2a)2=tg4acos4asin4a=tg4a