If i have a=exp x, and i know that my uncertainty in a is 3% ( at all

1. Nov 21, 2013

lavster

if i have a=exp x, and i know that my uncertainty in a is 3% ( at all points). At a point where a=2, how do we calculate the uncdrtainty in x.

is the absolute value = (3% of ln 2)/ln 2, which equals 0.03mm, which equals 1% (if x is 3mm)

thanks

2. Nov 21, 2013

Staff: Mentor

x cannot have units of mm in that equation, and a=2 does not correspond to x=3.

Use error propagation - write the expression as x=ln(a), the derivative with respect to a is easy to find then.
The formula looks good, but the result has the problem mentioned above.

3. Nov 21, 2013

lavster

so dx = da/a?

so the absolute uncertainty in x is equal to the percentage uncetainty in a?

4. Nov 21, 2013

Right.