# Thread: Calculating relative and absolute error

1. ## Calculating relative and absolute error

A =5 , B =4.8 and C = 0.2. The relative error on A, B and C is 1%. Calculate the absolute and relative error on D when D is calculated as

D =A/C + B/C

Explain why the relative error in D is large.

I'm new to this topic and got a little confused, we don't get any solutions so here is my attempt.

ABSOLUTE D = l0.5/0.02l + l-0.048/0.02l = 27.4

RELATIVE D = 0.02/0.5 + 0.03083/0.5 = 0.0816

I calculated the relative error of A, B and C using the formula lerror/valuel

I know these will most likely be wrong but could someone give me guidance on how I might correct these?

Thanks

2. ## Re: Calculating relative and absolute error

It looks like you put the absolute and relative errors for A, B, and C into the formula for D. Why in the world did you do that?

If the measured value for A was 5 with 1% relative error, the absolute error was plus or minus 0.05 so A could be as large as 5.05 or as low as 4.95. Similarly, the measured value for B was 4.8 with 1% error so B could be as large as 4.8+ 0.048= 4.848 or as low as 4.8- 0.048= 4.752. Finally, the measured value for C was 0.2 with 1% error so C could be as large as 0.2+ .0002= 0.2002 or as low as 00.2- 0.002= 0.198. The value for D could be as large as (5.05+ 4.848)/0.198= 50 or as low as (4.9+ 4.752)/2.002= 48.2. (5+ 4.8)/0.2= 49 so the absolute error in D is 1.00 and the percentage error is 1/49 or about 2%.

3. ## Re: Calculating relative and absolute error

AH! that helped me understand what I'm supposed to be doing a lot better! thank you!