Question

If $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$, then the value of $u$ can be written as

• A. $\frac{f}{1-uv}$
• B. $\frac{vf}{f-v}$
• C. $\frac{1}{v-f}$
• D. $\frac{f}{vf-1}$

Answer 1

The correct option is B $\frac{vf}{f-v}$

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

$\frac{1}{u}=\frac{1}{v}-\frac{1}{f}$

$\frac{1}{u}=\frac{f-v}{vf}$

$\therefore u=\frac{vf}{f-v}$