Опыт

Verified answer

Cos(a-b)/cos(a+b)=(cosacosb+sinasib)/(cosacosb-sinasinb)=
=(cosacosb/sinasinb+1)/(cosacosb/sinasinb-1)=(ctgactgb+1)/(ctgactgb-1)
sin(a+b)+sin(-a)cos(-b)=sinacosb+sinbcosa-sinacosb=sinbcosa
cos(-a)sin(-b)-sin(a-b)=-cosasinb-sinacosb+sinbcosa=-sinacosb