1.
(sina)^2+(sinb)^2-(sinasinb)^2+(cosacosb)^2
=(sina)^2-(sinasinb)^2 + 1-(cosb)^2+(cosacosb)^2
=(sina)^2[1-(sinb)^2] - (cosb)^2[1-(cosa)^2] +1
=(sinacosb)^2-(cosbsina)^2+1
=0+1
=1
2.
左边-右边
=2(1-sina+cosa-sinacosa)-[1+(sina)^2+(cosa)^2 -2sina+2cosa-2sinacosa]
=2(1-sina+cosa-sinacosa)-(2-2sina+2cosa-2sinacosa)
=0
3.
左边=[(sina/cosa)^2-(cosa/sina)^2]/[(sina)^2-(cosa)^2]
=[(sina)^2+(cosa)^2]/[(cosa)^2(sina)^2]
=(seca)^2+(csca)^2