Сократите дробь (1+a^2/3):(1-a^4/3)
(1+a² / 3) : (1-a^4 / 3) = (1+a² / 3) : ( (1-а²)(1+а²) / 3) = (1+a² / 3) * (3 / (1-а²)(1+а²) ) = 1 / (1-а²)