A concave lens with power -7 D is coaxially placed adjacent to a convex lens with power + 12 D. Calculate the focal length of combined lens.

Explanation

To solve the problem of finding the focal length of a combined lens system consisting of a concave lens and a convex lens, we can follow these steps:

### Step 1: Identify the Powers of the Lenses
- The power of the concave lens (P1) is given as -7 D (diopters).
- The power of the convex lens (P2) is given as +12 D.

### Step 2: Calculate the Net Power of the Combined Lens
- The net power (P) of the combined lens system is the sum of the powers of the individual lenses:
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