When written in usual form 100a + 10c + 9 is equal to ________.

Answer

To solve the problem, we need to express the given expression \(100a + 10c + 9\) in its usual numerical form.

### Step-by-Step Solution:

1. **Identify the components of the expression**:
- The expression consists of three parts: \(100a\), \(10c\), and \(9\).
- Here, \(a\) represents the digit in the hundreds place, \(c\) represents the digit in the tens place, and \(9\) represents the digit in the units (ones) place.

2. **Write the expression in standard numerical form**:
- The term \(100a\) indicates that the digit \(a\) is in the hundreds place. Therefore, it contributes \(100 \times a\) to the overall number.
- The term \(10c\) indicates that the digit \(c\) is in the tens place. Therefore, it contributes \(10 \times c\) to the overall number.
- The term \(9\) is in the ones place, contributing \(9\) to the overall number.

3. **Combine the contributions**:
- When we combine these contributions, we get the usual form of the number as:
\[ 100a + 10c + 9 \]

4. **Express the number in the usual form**:
- The usual form of the number can be represented as \(AC9\), where \(A\) is the digit in the hundreds place, \(C\) is the digit in the tens place, and \(9\) is the digit in the ones place.

Thus, when written in usual form, \(100a + 10c + 9\) is equal to \(AC9\).

### Final Answer:
When written in usual form, \(100a + 10c + 9\) is equal to **\(AC9\)**.