add additional contents for chp09

Author: longfei-chen

chp09/subsections/9.5.html

@@ -676,6 +676,12 @@ <h6>隐函数求导公式（一个方程）</h6>
                         $$\frac{\mathrm{d}y}{\mathrm{d}x} = -\frac{F_x}{F_y}$$
                     </div>
                     <p>前提条件：$F_y \neq 0$</p>
+                    <p>对于三元函数 $F(x,y,z) = 0$</p>
+                    <div class="math-expression">
+                        $$\frac{\partial{z}}{\partial{x}} = -\frac{F_x}{F_z}$$
+                        $$\frac{\partial{z}}{\partial{y}} = -\frac{F_y}{F_z}$$
+                    </div>
+                    <p>前提条件：$F_z \neq 0$</p>
                 </div>
 
                 <div class="theorem-box">