Multiplying Decimals by Negatives

Date: 03/18/2002 at 21:25:47
From: Miki
Subject: How to multiply decimals by negatives

How do you multiply negatives by decimals? I can't find decimals on my 
calculator. My exact problem is .56 times -50. Is there even an 
answer?

Date: 03/19/2002 at 09:43:17
From: Doctor Rick
Subject: Re: How to multiply decimals by negatives

Hi, Miki.

Do you know how to multiply 56 by -50? You multiply 56 by 50, then 
give the answer a negative sign because the signs of the two factors 
are different. If the signs were the same (either both positive or 
both negative), the product would be positive.

  56 * 50 = 2800
  56 * -50 = -2800

Do you know how to multiply 0.56 by 50? First multiply 56 by 50, then 
move the decimal point two places to the left, because that's what you 
do to change 56 to 0.56. If both factors had digits to the right of 
the decimal point, you'd move the decimal point of the product to the 
left as many places as there were digits to the right of the decimal 
points in the two factors combined.

  56 * 50 = 2800
  0.56 * 50 = 28.00

We can do both at once. To multiply 0.56 by -50, you first multiply 56 
by 50. Then you put a negative sign on the product, AND you move the 
decimal point two places to the left.

  56 * 50 = 2800
  0.56 * -50 = -28.00

Now, about the calculator. I have never seen a calculator (unless it 
was one we made in a programming class) that did not have a decimal 
point. Just enter

  . 5 6

The negative number may be tricky. Many computers don't let you enter

  - 5 0

in that order to enter a negative number. Instead you have to enter

  5 0 +/-

That +/- is a single key that changes the sign of the number in the 
display. After you press it, the display will read -50. Thus the 
complete calculation is entered as

  . 5 6 * 5 0 +/- =

on most calculators I have seen. (* is the multiplication sign.)

- Doctor Rick, The Math Forum
  http://mathforum.org/dr.math/