A metal pipe with outer radius of 10cm ,inner radius of 8cm and length of 20 have allowed errors of 1cm, 0.5cm and 2cm respectively. If need to estimate the worst-case error in volume.

So I know that volume = (pi*r1^2*l) -(pi*r2^2*l)

Then using partial differentiation I got

V_{r1}= 2pi*r1*l

_{Vr2}= -2pi*r2*l

V_{l}= pi *r1^2 - pi*r2^2

V(10,8,20) = 720pi

is this the correct formula to calculate error

abs(e) = abs(V_{ri}*E1) +abs(V_{r2}*E2) +abs(Vl *E3)

Im not sure which error terms I should sub in, should i convert allowed error into a decimal or simply sub in as they are?

Any help would be great