The integrand is \(x^2\), so we integrate it with respect to \(x\).
被积函数是 \(x^2\),所以我们对 \(x\) 进行积分。
To evaluate the integral, we first rewrite the integrand using a trigonometric identity, which makes the computation much simpler.
为了计算这个积分,我们先用一个三角恒等式改写被积函数,这会让运算简单得多。