sin^2x-sin2x=3cos^2x
sin^2x-2sinxcosx=3cos^2x /cos^2x
tg^2x-2tgx-3=0
a^2-2a-3=0
D=4+12=16=4^2
x1=3; x2=-1;
tgx=3
x=arctg 3+ Пn
tgx=-1
x= -П/4+Пn
