Update lecture.md

course/lecture/lecture.md

+<script src="//yihui.org/js/math-code.js"></script>
+<!-- Just one possible MathJax CDN below. You may use others. -->
+<script async
+  src="//mathjax.rstudio.com/latest/MathJax.js?config=TeX-MML-AM_CHTML">
+</script>
 
+$x_i = 1$
+
+$$ 
+    \left\{ 
+    \begin{array}{l}
+        \displaystyle J(u)  := \displaystyle \int_0^{t_f} L(x(t),u(t)) \, \mathrm{d}t \longrightarrow \min \\[1.0em]
+        \dot{x}(t) =  f(x(t),u(t)), \quad  u(t) \in U, \quad t \in [0, t_f] \text{ a.e.}, \\[1.0em]
+        x(0) = x_0 , \quad c(x(t_f)) = 0_{\mathrm{R}^k},
+    \end{array}
+    \right. 
+$$