79б
(1-cosa+1+cos)/(1-cos²a)=2/sin²a
79в
сos²a/sin²a *(-sin²a)+1=-cos²a+1=sin²a
79г
(tgb+1):(1+1/tgb)=(tgb+1):(tgb+1)/tgb=(tgb+1)*tgb/(tgb+1)=tgb
80а
(sin²a+cos²a+2sinacosa)/(sin²a+2sinacosa+cos²a)=1
80б
(sin²b-cos²b+sin²b+cos²b)/sin²b=2sin²b/sin²b=2
80в
1/(1+tg²b+1:(1+1/tg²b)=1/(1+tg²b)+1:[(tg²b+1)/tg²b]=
=1/(tg²b+1)+tg²b/(tg²b+1)=(1+tg²b)/(1+tg²b)=1
80г
(1+sinb)(1-sinb)/(cosb*cosb)=(1-sin²b)/cos²b=cos³b=1