diff --git a/15曲面的表示与逼近.tex b/15曲面的表示与逼近.tex
index 46e259f..6a09cda 100644
--- a/15曲面的表示与逼近.tex
+++ b/15曲面的表示与逼近.tex
@@ -49,7 +49,7 @@
 \[\bvec{n} \parallel \dfrac{\partial \bvec{r}}{\partial u}(u_0, v_0) \times \dfrac{\partial \bvec{r}}{\partial v}(u_0, v_0)\]
 
 进一步假设$\dfrac{\partial \bvec{r}}{\partial u}(u_0, v_0) \times \dfrac{\partial \bvec{r}}{\partial v}(u_0, v_0) \neq \bvec{0}$即两向量不共线，则可取$S$在$P$点法向$\bvec{n} = \dfrac{\partial \bvec{r}}{\partial u}(u_0, v_0) \times \dfrac{\partial \bvec{r}}{\partial v}(u_0, v_0)$，得到切平面方程
-\[\brak{\dfrac{\partial \bvec{r}}{\partial u}(u_0, v_0) \times \dfrac{\partial \bvec{r}}{\partial v}(u_0, v_0), \bvec{r} - \bvec{r}_0}\]
+\[\brak{\dfrac{\partial \bvec{r}}{\partial u}(u_0, v_0) \times \dfrac{\partial \bvec{r}}{\partial v}(u_0, v_0), \bvec{r} - \bvec{r}_0} = 0\]
 利用行列式也可以得到切平面方程
 \[\begin{vmatrix}
     x - x_0 & y - y_0 & z - z_0\\