The fifth question. . I don't have any good methods, so I just calculated X = (- 1 root 13)/2.

Substituting in, the result is 0 or-4 3.

(If the calculation is correct ~ a little messy)

The sixth question seems to have been mentioned in our competition counseling, but I forgot to ask my classmates before telling you.

There is also a third question, which should be related to the cosine theorem ~ I suggest to check the math textbook of Grade Two first, otherwise it will be a little troublesome to understand ~ I'll think about how to do it.

I can't see some questions clearly ~ even if I can see them clearly, I can't do them for a while ~

I will go to school soon, and I will continue to study.