31. Every camera lens has a measurement called the focal length, f, such that when an object is in focus, the distance from the object to the center of the lens, Do, and the distance from the center of the lens to the film, Di, satisfy the equation . If the object is in focus, Do = 36 centimeters, and Di = 12 centimeters, what is the focal length of the lens, in centimeters?

Answer and Explanation

Your Answer is

Correct Answer is C

Explanation

Bring the values ​​of the two Ds into the original formula, and find the answer that F is equal to 9. Choose C.