68. (1 - cos 2alpha + sin 2alpha) / (1 + cos 2alpha + sin 2alpha) = ctg alpha = 1;

(1 - cos 2alpha + sin 2alpha) / (1 + cos 2alpha + sin 2alpha) = (2 sin^2 alpha + 2 sin alpha cos alpha) / (2 cos^2 alpha + 2 sin alpha cos alpha) = (2 sin alpha (sin alpha + cos alpha)) / (2 cos alpha (cos alpha + sin alpha)) = tg alpha.

tg alpha = 1 implies alpha = pi/4 + n*pi.