Hi, I was reading a section which explains about the(adsbygoogle = window.adsbygoogle || []).push({});

"thin lens equation for diverging lenses" and it included an example;

Formula:

1/f = 1/di + 1/do

where f is the focal distance,

di is the distance of the image from the center and,

do is the distance of the object from the center.

Example:

f = -4cm

do = 3cm

di = ???

1/di = 1/f - 1/do

1/di = -1/4cm - 1/3cm

1/di = -3/12cm - 4/12cm

1/di = -7/12cm

1/di = -12/7cm = -1.7cm

The part where I have a confusion is colored in red.

I've been questioning, why -7/12cm was flipped in the last step.

If that example was to be solved as decimal numbers it would be;

1/di = -0.250cm^-1 - 0.333cm^-1

1/di = -1/0.583cm

then in this case, I would understand why the number has been reversed because it was ^-1

but when solving in fraction, I get confused..

Could anybody please explain for me?

Thank you in advance

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# Homework Help: Thin lens equation for diverging lenses.

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