$$ \int_{T/4}^0 - \int_{T/4}^{T/2} \quad = \quad \int_{T/4}^0 + \int^{T/4}_{T/2} \quad = \quad \int_{T/2}^0 $$
In general, $$ \int_a^b+\int_b^c=\int_a^c. $$
$$ \int_{T/4}^0 - \int_{T/4}^{T/2} \quad = \quad \int_{T/4}^0 + \int^{T/4}_{T/2} \quad = \quad \int_{T/2}^0 $$
In general, $$ \int_a^b+\int_b^c=\int_a^c. $$