76. (2 cos 2alpha - sin 4alpha) / (2 cos 2alpha + sin 4alpha) = tg^2 (pi/4 - alpha);

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

(1 - sin 2alpha) / (1 + sin 2alpha) = (cos alpha - sin alpha)^2 / (cos alpha + sin alpha)^2 = ((cos alpha - sin alpha) / (cos alpha + sin alpha))^2 = ((1 - tg alpha) / (1 + tg alpha))^2 = tg^2 (pi/4 - alpha).