Now, we'd like to be able to state the error in combinations like A+B or A-B or AB or A/B. This is done as follows:
(A ± a) + (B ± b) = (A+B) ± (a+b), and,
(A ± a) - (B ± b) = (A-B) ± (a
(A ± a) × (B ± b) = AB × [1 ± (a/A + b/B)].
You can equally well combine percentage errors instead of "relative" errors... a percentage error of 10% is a relative error of 0.1.
So if the percentage error in A is 10% and in B is 25%, the percentage error in A*B or B/A or A/B is 35%.
(A ± a) × (B ± b) = AB × [1 ± ( (a/A)2 + (b/B)2)0.5 ].