Update cheat-sheet_reconcile.md

cheat-sheet_reconcile.md

@@ -12,6 +12,13 @@ Ví dụ: [4,5,6] → **5** | [4,5,6,7] → (5+6)/2 = **5.5**
 **Mode:** Giá trị xuất hiện nhiều nhất  
 Ví dụ: [4,5,5,6] → **5**
 
+**Standard Deviation:** `σ = √[Σxi²/n - x̄²]`  
+Ví dụ: [4,7,10] → Σxi²=165, n=3, x̄=7  
+σ = √[165/3 - 7²] = √[55-49] = √6 = **2.45**
+
+**Standard Deviation:** `σ = √[Σxi²/n - x̄²]`  
+Ví dụ: [2,4,6] → Σxi²=56, x̄=4, n=3 → √[56/3 - 16] = √2.67 = **1.63**
+
 ---
 
 ### Boxplot
@@ -126,10 +133,10 @@ Ví dụ: (2,3), (4,5), (6,7) → **(4,5)**
 
 1. **Mean:** x̄ = Σxi/n
 2. **Median:** Giá trị giữa
-3. **Min-Max:** (x-min)/(max-min)×(b-a)+a
-4. **Z-Score:** (x-μ)/σ
-5. **Modified Z-Score:** 0.6745×(x-median)/MAD
-6. **σ:** √[Σxi²/n - x̄²]
+3. **σ (Std Dev):** √[Σxi²/n - x̄²]
+4. **Min-Max:** (x-min)/(max-min)×(b-a)+a
+5. **Z-Score:** (x-μ)/σ
+6. **Modified Z-Score:** 0.6745×(x-median)/MAD
 7. **Cov:** Σ(xi·yi)/n - x̄·ȳ
 8. **r:** Cov(X,Y)/(σx×σy)
 9. **Support:** Count/Total