原题 https://ti.luogu.com.cn/problemset/1038/training (T11)

积分结果 Link: https://www.wolframalpha.com/input?i2d=true&i=Integrate%5B%5C%2840%29x-y%5C%2841%29%2C%7By%2C0%2Cx%7D%2C%7Bx%2C0%2C1%7D%5D