math display fix for combination

diff --git a/docs/math/combination.md b/docs/math/combination.md
index 36b65a1..3494620 100644 (file)
--- a/docs/math/combination.md
+++ b/docs/math/combination.md
@@ -127,7 +127,7 @@ $$d_n = (n-1)(d_{n-2} + d_{n-1}) (n\geq 3)$$
 
 同时也有：
 
-$$d_n = n * d_{n-1} + (-1)^n$$
+$$d_n = n \times d_{n-1} + (-1)^n$$
 
 错位排列数列为 $0,1,2,9,44,265,\cdots$
 
@@ -153,4 +153,4 @@ $$C_n^0 + C_n^1 + C_n^2 + C_n^3 + \cdots + C_n^m = 2^n$$
 
 $$C_n^r + C_n^{r+1} = C_{n+1}^{r+1}$$
 
-$$\sum_{i=0}^m C_n^i C_m^{m-i} = C_{m+n}^m(n>=m)$$
+$$\sum_{i=0}^m C_n^i C_m^{m-i} = C_{m+n}^m(n \geq m)$$