Hi all! I've been working with differentiation for quite some time now, and I know how to use the product rule. f(x) = u(x)*t(x) f'(x) = u'(x)*t(x) + t'(x)u(x) But what I don't understand is how one derives it... Oh, and one more thing. How do we know a number is irrational, like pi and e ? Like, how do we know that it has an infinite number of decimals? Is it because when you do the decimal operation you will eventually see that there's a sequence of decimals which repeats itself, or not? Something else perhaps? Thanks in advance.