1)sin^4-cos^4=sin^2-cos^2;
sin^4 x-cos^4 x=
(sin^2 x+cos^2 x)(sin^2 x-cos^2 x)=
sin^2x- cos^2 x;
2)sina/(1-cosa)=(1+cosa)/sina;
sina/(1-cosa)=sina(1+cosa)/(1-cosa)(1+cosa)=
sina(1+cosa)/(1-cos^2a)=
sina(1+cosa)/sin^2 a=(1+cosa)/sina.
(sina-cosa)/(sina+cosa)=
(sina/sina-cosa/sina)÷(sina/sina+cosa/sina)=
(1-ctga)÷(1+ctga)
tga=2;
4sina-3cosa/5sina=4tga-3/tga=4*2-3/2=
5/2=2,5.
ctga=3;
4sina-3cosa/5sina=4-3ctga/5=
4-3*3/5=-5/5=-1.