Why do we only solve for $0$ on the numerator of this fraction?

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I have this function where I need to find the $x$-intercepts

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I see they are $x = 0, 4$. And know that this is found by doing: $x=0, (x-4) = 0$ [Solve].

But why is this only done for the numerator?

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If you're asking why we set only the numerator equal to 0 and not the denominator, recall that $0$ divided by any nonzero number is $0$. Similarly, any number divided by $0$ is undefined.

Therefore in your case, the fraction is only equal to $0$ when its numerator is equal to $0$ and it's denominator is not equal to $0$, as having a denominator equal to $0$ (which is akin to dividing by $0$) would render the expression undefined.