# Quick question about uncertainty calculation

1. Sep 29, 2008

### mjolnir80

1. The problem statement, all variables and given/known data
lets say we have A * B
A=12.8 + 0.2
B=3.75 + 0.08
how would we calculate the new uncertainty value
ive seen soo many ways of doing this im a bit confused

2. Relevant equations
any equation used to calculate uncertainty

3. The attempt at a solution
the 2 main ways ive seen is one involving partial dervatives and the other is taking the square root of the answer times the (uncertainties/value)2
im a little confused about which one to use

2. Sep 29, 2008

### rock.freak667

If C=AB, then

$$\frac{\delta C}{C}=\frac{\delta A}{A}+ \frac{\delta B}{B}$$

this is obtained using differential calculus.

3. Sep 29, 2008

### granpa

(A+ε1)*(B+ε2)
AB+Aε2+Bε11ε2

so if we disregard the final term the incertainty would be Aε2+Bε1

partial derivative of AB with respect to A is B
partial derivative of AB with respect to B is A

does that help?

4. Sep 29, 2008

### granpa

whats that called? I want to look that up. it looks interesting.

5. Sep 30, 2008

### granpa

Last edited by a moderator: Apr 23, 2017