gr. 11 math question

**colts** · September 16th 2008, 05:07 PM

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

**11rdc11** · September 16th 2008, 05:14 PM

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

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

**colts** · September 17th 2008, 01:46 PM

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

**11rdc11** · September 17th 2008, 01:51 PM

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

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

$is = f(i + s)$

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

**colts** · September 17th 2008, 01:56 PM

i still dun understand how they become 2 im so dumb

**11rdc11** · September 17th 2008, 02:04 PM

Find common denominator

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

so multiply the denominators together to find the common denominator

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

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