I'm working through the problems in Mary Boas's Mathematical Methods text. Here's how she began solving one problem... "We take differentials of the equation 1/i + 1/o = 1/f (f=constant) to get -di/i2 - do/o2 = 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.