Cos2a=1-2sin^2a=1-2*9/49=1-18/49=31/49
cosa=+-2sqrt(10)/7
tga=sina/cosa=-+3/(2sqrt(10))
ctga=-+2sqrt(10)/3
(sin2a*cosa)/(1+cosa)*2cos^2a=2sinacosa/(2cosa(1+cosa))=sina/2cos^2a/2=tga/2
x/3+10=-90+360k
x/3=-100+360k
x=-300+360*3k
x=П/3+2п(3k+1)
5x=п/2+Пk
x=П/10+Пk/5