4) (tga+tgb)/((1/tga)+(1/tgb))=tga*tgb
tga+tgb=(tga*tgb)((1/tga)+(1/tgb))
tga+tgb=tga*tgb*1/tga+1/tgb*tga*tgb
tga+tgb=tga+tgb
2) (cos^2)x*(cos^2)y-(sin^2)x*(sin^2)y=(cos^2)x-(sin^2)y
(cos^2)x*(cos^2)y-(cos^2)x-(sin^2)x*(sin^2)y+(sin^2)y=0
-(cos^2)x*[1-(cos^2)y]+(sin^2)y*[1-(sin^2)x]=0
-(cos^2)x*(sin^2)y+(cos^2)x*(sin^2)y=0
-(cos^2)x*(sin^2)y=-(cos^2)x*(sin^2)y /(умножим на -1)
(cos^2)x*(sin^2)y=(cos^2)x*(sin^2)y