[cos(a-π)/sin(π-a)]sin(a-π/2)cos(π/2+a)
=[-cosa/sina](-cosa)(-sina)
=-cos^2a还有一个，在下面~化简[cos(2π-a)sin(π+a)]/[sin(π/2+a)tan(3π-a)]=[ cosa(-sina)]/[cosa*tana]=-sina/tana=-cosa