In Example 3, solve for p.

Data from Example 3

An equation relating the focal length f of a lens with the object distance p and the image distance q is given below. See Fig. 6.4. Solve for q.

Because the only denominator is p + q, the LCD is also p + q. By first multiplying each term by p + q, the solution is completed as follows:

The last form is preferred because there is no minus sign before the fraction. However, either form of the result is correct.

f = pq P + q