karasikov 8cos^2_x + 2sinx - 7 = 0, 3cosx + 2√2 > 0;
1 - 8sin^2_x  + 2sinx = 0, cosx > -2√2 / 3; cosx > ~0.9;
(2sinx - 1)(4sinx + 1) = 0;
sinx = 1/2, sinx = -1/4;
x = Pi / 6 + 2Pi*n, 5Pi/ 6 + 2Pi*n, -arcsin(1/4) + 2Pi*n, -Pi + arcsin(1/4) + 2Pi*n;