diff --git a/chapter_optimization/optimization-intro.md b/chapter_optimization/optimization-intro.md
index 2781bee4c029b9fa9613707d5f8509bfb8424d3f..bb86509c1849822be3e72f1d9a4bdccf0f6b4f20 100644
--- a/chapter_optimization/optimization-intro.md
+++ b/chapter_optimization/optimization-intro.md
@@ -41,6 +41,7 @@ def f(x):
 
 d2l.set_figsize((4.5, 2.5))
 x = np.arange(-1.0, 2.0, 0.1)
+# 逗号表示只取返回列表中的第一个元素
 fig,  = d2l.plt.plot(x, f(x))
 fig.axes.annotate('local minimum', xy=(-0.3, -0.25), xytext=(-0.77, -1.0),
                   arrowprops=dict(arrowstyle='->'))