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1. 公式换行使等号对齐

   \begin{equation}
   \begin{aligned}
   Y&=2*(x+3)\\
   &=2*x+6
   \end{aligned}
   \end{equation}

$$\begin{equation} \begin{aligned} Y&=2*(x+3)\\ &=2*x+6 \end{aligned} \end{equation}$$

1. 联立方程组

   t^*=
   \left\{
   \begin{align} 
   x&=eq1 & \text{if x=1}\\
   y&=eq2 &\text{if x=2}
   \end{align}
   \right.

   $$t^*= \left\{ \begin{align} x&=eq1 & \text{if x=1}\\ y&=eq2 &\text{if x=2} \end{align} \right.$$