A convex lens of focal length 10 cm. is used as a magnifying lens.
Where should the object be placed if the image is to be 30 cm from the lens?

Explanation

To solve the problem, we need to find the object distance (u) for a convex lens given the focal length (f) and the image distance (v).

### Step-by-Step Solution:

1. **Identify the given values:**
- Focal length of the convex lens, \( f = 10 \, \text{cm} \) (positive for convex lenses).
- Image distance, \( v = -30 \, \text{cm} \) (negative because the image is on the same side as the object in a magnifying setup).

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