Log2_x + logx_2 = -2;
log2_x + 1/ log2_x = -2;
log2_x = t;
t + 1/t = - 2;
t^2 + 2t + 1 = 0;
D = 4-4= 0;
t = -2;
log2_x = -2;
log2_x = log2_(1/4);
x = 1/4.
cos b= sgrt2/4;
cos 2b = 2 cos^ b - 1 = 2*(sgrt2/4)^2 - 1 = 2*2/16 - 1 = 1/4 - 1 = - 0,75;
0,15 - cos 2b= 0.15 - (-0,75) = 0,15 + 0,75 = 0,9