316-319 316 Докажи уравнение 317 покажи через ctg

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.