diff --git a/advanced-math/exercise/4-integal-of-functions-of-single-variable/integal-of-functions-of-single-variable.pdf b/advanced-math/exercise/4-integal-of-functions-of-single-variable/integal-of-functions-of-single-variable.pdf
index 25228b1..a1bc565 100644
Binary files a/advanced-math/exercise/4-integal-of-functions-of-single-variable/integal-of-functions-of-single-variable.pdf and b/advanced-math/exercise/4-integal-of-functions-of-single-variable/integal-of-functions-of-single-variable.pdf differ
diff --git a/advanced-math/exercise/4-integal-of-functions-of-single-variable/integal-of-functions-of-single-variable.tex b/advanced-math/exercise/4-integal-of-functions-of-single-variable/integal-of-functions-of-single-variable.tex
index 05e7722..de1194b 100644
--- a/advanced-math/exercise/4-integal-of-functions-of-single-variable/integal-of-functions-of-single-variable.tex
+++ b/advanced-math/exercise/4-integal-of-functions-of-single-variable/integal-of-functions-of-single-variable.tex
@@ -741,6 +741,7 @@ $\int_0^\frac{\pi}{2}e^{2x}\cos x\,\textrm{d}x=\dfrac{1}{5}[e^{2x}(\sin x+2\cos
     \item 先提出$\dfrac{1}{n}$。
     \item 凑出$\dfrac{i}{n}$。
     \item 写出$\int_0^1f(x)\,\textrm{d}x$，其中$\dfrac{1}{n}$没有了，将所有$\dfrac{i}{n}$换为$x$。
+    \item 将$n$消去，如将$n$极限归为1。
 \end{enumerate}
 
 \textbf{例题：}求$\lim\limits_{n\to\infty}\left(\dfrac{1}{n+1}+\dfrac{1}{n+2}\cdots+\dfrac{1}{n+n}\right)$。