# Homework Help: Geomertical optics- derivation of graph of (u+v) against u

1. Aug 23, 2014

### somecelxis

1. The problem statement, all variables and given/known data

may i know how to derive the the sha[pe of graph and get the point of min (2f , 4f) ? i really have no idea how to get the shape of the graph and the min point. The book give it without any explaination.

2. Relevant equations

3. The attempt at a solution

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2. Aug 23, 2014

### Simon Bridge

Do you have an equation that related u,v, and f?
Put v=d-u, and solve for d.

3. Aug 23, 2014

### somecelxis

what equation is that? v=d-u i have only 1/u +1/v =1/f in my book

4. Aug 23, 2014

### Simon Bridge

1/u + 1/v = 1/f is a good place to start - that's the lensmakers formula.

can you rearrange that equation to make v the subject?

5. Aug 23, 2014

### somecelxis

v= uf /(u-f) what shall i do next?

6. Aug 23, 2014

### Simon Bridge

OK - so if that is v, then v+u = ?