PSAT Math Multiple-Choice Question 716: Answer and Explanation

Question: 716

In physics, the mirror equation is , where f represents the mirror's focal length, d_o is the distance of an object from the mirror, and d_i is the distance of the image from the mirror. Which expression gives d_o in terms of focal length and image distance?

• A.
• B.
• C.
• D.

Correct Answer: D

Explanation:

(D) First, isolate by subtracting from both sides of the equation:

We want to find d_o, so we need the reciprocal of , which is d_o. To find the reciprocal, simply take 1 over both sides:

The answer is choice (D).