2,1a
-sin²a/cos²a -1=-tg²a-1=-1/cos²a
2.1b
2cosπ/3cosa-2sinπ/3sina-cosa+√3sina=cosa-√3sina-cosa+√3sina=0
2.1c
2sin4acos2a/2cos4acos2a=tg4a
2.2
cos(270-30)-ctg(180-45)=-sin30+ctg45=-1/2+1=1/2
2.3
sina=-5/13⇒cosa=-√(1-sin²a)=-√(1-25/169)=-√(144/169)=-12/13
cosb=-12/13⇒sinb=√(1-cos²b)=√(1-144/169)=√(25/169)=5/13
cos(a-b)=cosacosb-sinasinb=(-12/13)²-(5/13)²=144/169-25/169=119/169
2.4a
sinx/cosx-cosx/(1-sinx)=(sinx-sin²x-cos²x)/(cosx(1-sinx))=
=(sinx-1)/cosx(1-sinx)=-1/cosx
2.4b
(cosa-cos5a)*sin3a/(1-cos6a)=2sin3asin2a*sin3a/2sin²3a=sin2a