# Relationship between object distance, image distance, focal distance for doubly convex lenses

#### j_limb14

##### New member
Problem:

So, I finished part A of this question, but I need help with finishing part B.

#### Harry_the_cat

##### Senior Member
$$\displaystyle \frac{1}{a}+\frac{1}{b} \neq \frac{1}{a+b}$$
You add fractions by first getting a common denominator.