Provide an Appropriate Response $$f = \frac { a b } { b + a }$$

Multiple Choice

Provide an appropriate response.
-A formula for the focal length of a lens is $$f = \frac { a b } { b + a }$$ Calculate f (the focal length) for a = 8 cm and b = 17 cm. Round to the nearest tenth.

A) 2.6 cm
B) 4.0 cm
C) 8.5 cm
D) 5.4 cm