I am curious why the area $\Delta p \Delta q$ is so small that it can safely be neglected.
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It says "for small values of delta p and delta q". So something small times something small is something very small. Basically we are taking a limit and as delta p and delta q go towards zero, the product goes to zero faster.
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$\begingroup$ The thing is, we have used this in examples regarding not only marginal, but greater changes, so that's what makes me confused. $\endgroup$ – faljipx Dec 7 '17 at 23:54