The relationship of focal length, object to lens and lens to image distance shown below. $$\frac{1}{f} = \frac{1}{a} + \frac{1}{b}$$ Here $f$ is focal length, $a$ is lens to object distance, $b$ is lens to image distance. From the equation above the focal length is $$ f = \frac{ab}{a + b} $$ Measure up to 5 set of $a$, $b$ and fill the table below.

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