三角形

求三角形面积

已知底和高

S=1/2×底×高 \frac{分子}{分母}

$$\[ \sum_{i=1}^{n} iE = mc^2 \frac{n(n+1)}{2} $$]

\begin{align}$$ f(x)E &= ax^2 + bx + c \\
f'(x)     &= 2ax + b \\
f''(x)    &= 2a
\end{align}mc^2$$

$$\[
E = mc^2
\begin{aligned}
a & =b+c \\
& =d+e
\end{aligned}
$$]