# Thread: [SOLVED] Need help understanding this question.

1. ## [SOLVED] Need help understanding this question.

$\displaystyle \frac {\sigma_z}{z} = \frac {\sigma_x}{x} + \frac {\sigma_y}{y}$

For z=x+y
where x=30cm +/- 5cm
and y=40cm +/- 8cm

I don't understand what to do here, I guess I don't know what they are asking. How does this get set up?

How do I do the z, x, and y parts? Do I need to do it 2x, once for + and another for -?

2. Originally Posted by redman223
$\displaystyle \frac {\sigma_z}{z} = \frac {\sigma_x}{x} + \frac {\sigma_y}{y}$

For z=x+y
where x=30cm +/- 5cm
and y=40cm +/- 8cm

I don't understand what to do here, I guess I don't know what they are asking. How does this get set up?

How do I do the z, x, and y parts? Do I need to do it 2x, once for + and another for -?
Substitute x = 30, $\displaystyle \sigma_x = 5$, y = 40, $\displaystyle \sigma_y = 8$, z = x + y = 70.

Solve for $\displaystyle \sigma_z$.

It's expected you've got some familiarity with error analysis.