[問題] 數學式出問題

Your math has many errors simply look at it.

Try 
\begin{eqnarray}
.....&=&....\\
.....&=&....\\
\end{eqnarray}
Without any $$ in the above environment.

Have you read any beginner's latex?
Plenty of them, all are free at your finger tips.
Try Latex basic, latex for beginners.

推 maximum90123:謝謝大大熱心幫忙,但我用這方法貼我本來的方程式
→ maximum90123:會多一行空白的方程式出來耶...不過已經不會跑出錯誤
→ annheilong:最後一行方程式的最後面不要有 \\ 換行試試看?
推 maximum90123:可以了,謝謝