I'm working through the problems in Mary Boas's Mathematical Methods text. Here's how she began solving one problem...(adsbygoogle = window.adsbygoogle || []).push({});

"We take differentials of the equation 1/i + 1/o = 1/f (f=constant) to get

-di/i^{2}- do/o^{2}= 0."

So on the left side the first term was differentiated with respect to i and the second term was differentiated with respect to o. Why is it okay to differentiate these terms by different variables? I'm only used to differentiating by one variable at a time.

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# B Differentiating by two different variables -- when is it okay?

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