5sin^2x - 2sinxcosx + cos^2x=4
5sin^2x - 2sincosx + cos^x=4.(sin^2x + cos^2x)
5sin^2x - 2sincosx + cos^x=4sin^2x + 4cos^2x
5sin^2x - 2sincosx + cos^x - 4sin^2x - 4cos^2x=0
sin^2x - 2sincosx -3cos^2x=0 | : cos^2x
tg^2x - 2tg^2x - 3=0
tgx=t
t^2 - 2t^2x - 3=0
D= 4+12=16
t1=-2+4/2= 1
t2=-2-4/2=-3
tgx =1
x= П/4 + Пn, n - целое число
tg = -3x= -arctg3+ Пn,
n- целое число