$\pi \int_{0}^{1} (x^2 - x^4) dx$

$\pi \int_{0}^{1} (x - x^2)^2 dx$

$\pi \int_{0}^{1} (x^2 - x^2)^2 dx$

$\pi \int_{0}^{1} (x^2)^2 - (x)^2 dx$