$f\left( \left[ \frac{ 1+\left\{x,y\right\} }{ \left( \frac{x}{y}+\frac{y}{x} \right) \left(u+1\right) }+a \right]^{3/2} \right) \tag{行标}$

$f(x + \epsilon) \approx f(x) + f'(x) \epsilon + \mathcal{O}(\epsilon^2).$

$\smash{\displaystyle\max_{0 \leq q \leq n-1}} f(q) \le n$

$\LaTeX$ 好评！！！！！