Sunday, June 06, 2010

GED Math & Subtracting Negative Integers

Perhaps one of the most difficult concepts for learners to comprehend is negative integers. If you show them a problem such as 7-4, they know that the answer is 3.

But if you give them 7-(-4), they may still think the answer is 3.

So what is the best way to teach subtraction of integers, both positive and negative?

I have found that if you teach the basic method of "adding the opposite", that will help.

Example: 7-(-4)= ??

7 + (4) = 11

Step one: Keep the first integer 7
Step two: Change the negative sign to a positive sign
Step three: Change the sign of the second integer. In this case, The opposite of negative four is positive four.
Step four: Add the two integers.

1 comment:

David said...

One meaning of subtraction is "take away", but another meaning is "how many more?" For example, 5 - 3 = 2 says that 5 is 2 more than 3. Similarly, 7 - (-4) = 11 because 7 is 11 more than -4.

Ask students to figure out 7 - (-4) by plotting 7 and -4 on a number line, and counting from -4 to 7. Give real world examples. If the low temperature was 4 degrees below zero, and the high temperature was 7 degrees above zero, by how much did the temperature increase?

Show students the double-negative rule after they have experienced subtracting with a number line, or let them discover the rule for themselves.