diff --git a/advanced-math/exercise/6-differential-calculus-of-multivariate-functions/differential-calculus-of-multivariate-functions.pdf b/advanced-math/exercise/6-differential-calculus-of-multivariate-functions/differential-calculus-of-multivariate-functions.pdf
index 4ee7ca7..dadcc04 100644
Binary files a/advanced-math/exercise/6-differential-calculus-of-multivariate-functions/differential-calculus-of-multivariate-functions.pdf and b/advanced-math/exercise/6-differential-calculus-of-multivariate-functions/differential-calculus-of-multivariate-functions.pdf differ
diff --git a/advanced-math/exercise/6-differential-calculus-of-multivariate-functions/differential-calculus-of-multivariate-functions.tex b/advanced-math/exercise/6-differential-calculus-of-multivariate-functions/differential-calculus-of-multivariate-functions.tex
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--- a/advanced-math/exercise/6-differential-calculus-of-multivariate-functions/differential-calculus-of-multivariate-functions.tex
+++ b/advanced-math/exercise/6-differential-calculus-of-multivariate-functions/differential-calculus-of-multivariate-functions.tex
@@ -92,7 +92,7 @@ $\dfrac{\partial^2u}{\partial y^2}=\dfrac{\partial}{\partial y}\left(\dfrac{\par
 
 \paragraph{特殊值反代} \leavevmode \medskip
 
-若是给出的不等式后还给出对应的特殊值，可以直接代入然后反代求出函数，而不用链式法则。
+若是给出的不等式后还给出对应的特殊值，可以直接代入然后反代求出函数，而不用链式法则。这里一般只能当一个变量为0才能带入，因为0与其他数运算后不变。
 
 \textbf{例题：}设$z=e^x+y^2+f(x+y)$，且当$y=0$时，$z=x^3$，则求$\dfrac{\partial z}{\partial x}$。