# Math Help - gr. 11 math question

1. ## gr. 11 math question

gr. 11 math questions

a cameras focal lengt, f, is relatedto the distance from the subject, s, and the image distance, i ,by the equation 1/s + 1/i = 1/f. find a formula to express f in terms of s and i.

i have no idea watsoever wat to do

2. If I understand your question correctly it wants you to solve for f

$f= \frac{is}{i + s}$

3. huh how did u get that?? can u tell me how u got 2 i's and s's

4. $\frac{1}{s} + \frac{1}{i}= \frac{1}{f}$

$\frac{i + s}{is} = \frac{1}{f}$

$is = f(i + s)$

$\frac{is}{i + s} = f$

5. i still dun understand how they become 2 im so dumb

6. Find common denominator

$\frac{1}{s} + \frac{1}{i}$

so multiply the denominators together to find the common denominator

$\frac{(1)(i) + (1)(s)}{is}$