curve（pf（x1，2）（0.2））pf里的数字相对都代表着什么？
自由度https://vimsky.com/examples/usage/compute-the-value-of-f-cumulative-distribution-function-in-r-programming-pf-function.html#:~:text=pf%20%28%29%20R,%E8%AF%AD%E8%A8%80%E4%B8%AD%E7%9A%84%E5%87%BD%E6%95%B0%E7%94%A8%E4%BA%8E%E8%AE%A1%E7%AE%97%20F%20%E7%B4%AF%E7%A7%AF%E5%88%86%E5%B8%83%E5%87%BD%E6%95%B0%E5%9C%A8%E4%B8%80%E7%B3%BB%E5%88%97%E6%95%B0%E5%80%BC%E4%B8%8A%E7%9A%84%E5%AF%86%E5%BA%A6%E3%80%82
