Two forces of magnitudes 3N and 4N respectively are acting on a body. Calculate the resultant force if the angle between them is-
`180^(@)"`

Explanation

To solve the problem of finding the resultant force when two forces of magnitudes 3N and 4N are acting on a body at an angle of 180 degrees, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Forces**:
- Let \( F_1 = 3 \, \text{N} \) (the first force)
- Let \( F_2 = 4 \, \text{N} \) (the second force)

2. **Determine the Angle**:
- The angle \( \theta \) between the two forces is given as \( 180^\circ \).

3. **Use the Formula for Resultant Force**:
- The formula for the resultant force \( R \) when two forces are acting at an angle \( \theta \) is given by:
\[ R = \sqrt{F_1^2 + F_2^2 + 2F_1F_2 \cos \theta} \]

4. **Substitute the Values**:
- Since \( \theta = 180^\circ \), we know that \( \cos 180^\circ = -1 \).
- Substitute \( F_1 \), \( F_2 \), and \( \cos \theta \) into the formula:
\[ R = \sqrt{3^2 + 4^2 + 2 \cdot 3 \cdot 4 \cdot (-1)} \]

5. **Calculate Each Term**:
- Calculate \( F_1^2 = 3^2 = 9 \)
- Calculate \( F_2^2 = 4^2 = 16 \)
- Calculate \( 2F_1F_2 \cos \theta = 2 \cdot 3 \cdot 4 \cdot (-1) = -24 \)

6. **Combine the Results**:
- Now substitute back into the equation:
\[ R = \sqrt{9 + 16 - 24} \]
- Simplify:
\[ R = \sqrt{25 - 24} = \sqrt{1} \]

7. **Final Result**:
- Thus, the resultant force \( R = 1 \, \text{N} \).

### Conclusion:
The resultant force when two forces of 3N and 4N are acting at an angle of 180 degrees is **1N**.