For a question like In an experiment, the external diameter D and internal diameter d of a metal tube were found to be (64 +/- 2) mm and (47 +/- 1) mm respectively. What is the maximum percentage error for the cross-sectional area of the metal tube? I will need to find the external area. So, area = pi(1/2d)^2 But I'm confused over whether should the uncertainty change? What I mean was 1/2(64 +/- 2) = (32 +/- 1) ?? Or will the uncertainty remain at 2? If it remains at 2, the percentage uncertainty will definitely change. And why does the formula R=kAB, k is a constant and A and B are physical terms, has the uncertainty formula tR / R = tA / A + tB / B where tR / R, tA / A, tB / B are fractional uncertainty. Why is the constant excluded? R = kAB = AB + AB + AB ... + AB So won't the uncertainty add up? Thanks.