8cos²x-2sinxcosx-3=0
8cos²x-2sinxcosx-3*1=0
8cos²x-2sinxcosx-3(sin²x+cos²x)=0
8cos²x-2sinxcosx-3sin²x-3cos²x=0
5cos²x-2sinxcosx-3sin²x=0
5-2tgx-3tg²x=0
5-2t-3t²=0
3t²+2t-5=0
(t-1)(3t+5)=0
t1=1
t2=-5/3
tgx=1
tgx=-5/3
x1=П/4+Пn, n€Z
x2=arctg(-5/3)+Пk, k€Z