3cos^2x-sin^2+2sinx*cosx=0
3cos²x-sin²x+2sinxcosx=0/cos²x≠0 tg²x-2tgx-3=0 tgx=a a²-2a-3=0 a1+a2=2 U a1*a2=-3 a1=-1⇒tgx=-1⇒x=-π/4+πn,n∈Z a2=3⇒tgx=3⇒x=arctg3+πk,k∈Z