The main point is that Lu Zhou's method is easier than the answers! The answers didn't think about converting the Cartesian coordinates into polar coordinates. Instead, the answers used integral by parts which resulted in a large number of calculations.
It was the first time I saw her smile, and by God she was beautiful. When she smiled, it truly felt like time the whole world stopped and stared at her for a while.
"Sigh, you're really stubborn. Sometimes I hate your Emotion Control," Flourance said, sighing. He knew that nothing he said was going to change anything.