 It's very common. They were actually making more than one measurement and so a very common one would be something like addition so there you might have Two quantities that you're measuring and you're just adding them together. So in that case we can follow exactly the same process We're now we've allowed our derived quantity to depend on more than one variable and that means that when we're estimating our uncertainty we would have both This variable going to an extreme value and that value going to an extreme value And so what we've done down here is we've said it's just addition So I'm going to have a going to a plus delta a and we're going to have b going to b plus delta b and We're just taking away our average there And if we expand out the brackets the a cancels with the minus a and the b cancels with the minus b And we just get these two things added together So for addition you add the uncertainties