Solve, using properties/ methods that simplify the calculations:
`398xx9999`

Answer

To solve the problem \( 398 \times 9999 \) using properties that simplify calculations, we can follow these steps:

### Step-by-Step Solution:

1. **Rewrite the Numbers**:
- We can express \( 398 \) as \( 400 - 2 \).
- We can express \( 9999 \) as \( 10,000 - 1 \).
- So, we rewrite the expression:
\[ 398 \times 9999 = (400 - 2) \times (10,000 - 1) \]

2. **Apply the Distributive Property**:
- According to the distributive property, \( A \times (B - C) = A \times B - A \times C \).
- We can apply this property:
\[ (400 - 2) \times (10,000 - 1) = 400 \times 10,000 - 400 \times 1 - 2 \times 10,000 + 2 \times 1 \]

3. **Calculate Each Part**:
- Now, we calculate each term:
- \( 400 \times 10,000 = 4,000,000 \)
- \( 400 \times 1 = 400 \)
- \( 2 \times 10,000 = 20,000 \)
- \( 2 \times 1 = 2 \)

4. **Combine the Results**:
- Now, substitute the calculated values back into the expression:
\[ 4,000,000 - 400 - 20,000 + 2 \]

5. **Perform the Final Calculation**:
- First, subtract \( 400 \) from \( 4,000,000 \):
\[ 4,000,000 - 400 = 3,999,600 \]
- Next, subtract \( 20,000 \):
\[ 3,999,600 - 20,000 = 3,979,600 \]
- Finally, add \( 2 \):
\[ 3,979,600 + 2 = 3,979,602 \]

### Final Answer:
Thus, the solution to \( 398 \times 9999 \) is:
\[ \boxed{3,979,602} \]