Date: 02/25/99 at 04:14:53 From: Em Subject: Significant Figures I am having trouble with significant figures. I do not understand why: a) 62.3 multiplied by 5.7 = 360, but 62.30 multiplied by 5.70 = 355. The question says to express your answer with an appropriate number of significant figures. Please help me, as I do not understand why we get 360 and 355.
Date: 02/25/99 at 12:15:12 From: Doctor Peterson Subject: Re: Significant Figures The idea here is that if one of the numbers you are multiplying is only accurate to two significant digits, you can only trust two significant digits of the result, so you round to that accuracy. When the numbers being multiplied are given as 62.30 and 5.70, there are 4 and 3 significant digits respectively, so you can keep 3 digits in your answer, 355. But when you are only given 62.3 and 5.7, you should only keep 2 significant digits, so you round it up to 360. Here's one way to see why this is. (I'll use a different example, and explain why below.) The multiplication of 12.30 by 5.70 looks like this: 1 2.3 0 * 5.7 0 --------- 0 0 0 0 8 6 1 0 6 1 5 0 ----------- 7 0.1 1 0 0 If we don't know the last digit of each number, but represent each unknown digit by X, your multiplication looks like this: 1 2.3 X * 5.7 X --------- X X X X 8 6 1 X 6 1 5 X ------------- 7 0.X X X X I've written X wherever I don't know what a digit is, because I'm multiplying or adding an unknown digit. The X's show that I can't trust the last digits, and should round off to 70. If you look closely, you'll see that the significant digits in the answer come from the significant digits of the 5.7, the number with the fewest significant digits. So that's the rule: you keep as many significant digits in the product as there are in the factor with the fewest significant digits. Now here's your original problem: 6 2.3 X * 5.7 X --------- X X X X 4 3 6 1 X 3 1 1 5 X ------------- 3 5 5.X X X X You'll notice that it looks as if we have more valid digits than the rule says! That's because the first digits of both numbers are relatively large, so that you get an extra digit. The rule is an approximation, and is a little on the conservative side, assuming that it's better to keep too few digits than to trust too many in some cases. We could probably modify the rule slightly to take the extra digit into account, but the simple rule has been found to be good enough. The important thing, of course, is that whether you call the answer 355 or 360, you are rounding off several more digits from the actual product, 355.11, that are really meaningless. In this age of calculators, when you can get many digits in any calculation with no trouble, it is important not to keep all those digits and get a false sense of the precision of your results. We don't want to pretend we know seven digits when we really only know 2 or 3. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.