Express each of the following in the form b or bi, where b is a real number
`5i(-8i)`.

Answer

To express \( 5i(-8i) \) in the form \( b \) or \( bi \), where \( b \) is a real number, we can follow these steps:

### Step-by-Step Solution:

1. **Write the expression**: Start with the given expression.
\[ 5i(-8i) \]

2. **Multiply the coefficients**: Multiply the coefficients (5 and -8) and the imaginary units \( i \).
\[ 5 \cdot -8 \cdot i \cdot i = -40 \cdot i^2 \]

3. **Use the property of \( i^2 \)**: Recall that \( i^2 = -1 \).
\[ -40 \cdot i^2 = -40 \cdot (-1) \]

4. **Simplify the expression**: Simplify the expression by multiplying.
\[ -40 \cdot (-1) = 40 \]

5. **Final result**: The expression is now in the form \( b \).
\[ \text{Hence, the required answer is } 40. \]