The focus today was adding and subtracting decimal numbers using an input/output table. This was a little more involved than it might sound. To begin, we took a few notes in our journal to help us remember important ideas and new vocabulary. To create a page for your journal, please watch: Add and Subtract Decimals.
Basically, we reviewed that you must ALWAYS LINE UP DECIMAL POINTS when adding or subtracting. We discussed what to do when adding or subtracting whole numbers with decimal numbers.... just turn the whole number into a decimal number (see illustration above). Then we discussed some new vocabulary: minuend and subtrahend ....
The minuend is the number that is being subtracted FROM. The subtrahend is the number that is being SUBTRACTED. The kids had no trouble at all with difference, they have known and used that vocabulary word for years!
Once we finished with these notes we began the assignment, "Under the Weather," from The Mailbox resource Teacher's Helper Aug/Sept 2009. On this assignment, students were working with input/output tables.... a number goes in and a new number comes out using a rule. The rule was given to the students at the top of each table, but the confusion came when working backwards! For example, if the "in" number was given, the student followed the given rule to determine the "out" number. HOWEVER, if the "out" number was given, the student had to work backwards (do the opposite of the rule given). Once students understood the pattern, there was no longer any confusion! To view the instructions for the assignment, please watch: Under the Weather.
Finally, I asked the students to explain their understanding of adding and subtracting decimals by answering the following prompt:
I can conclude from today's activity that if I do not
line up the decimal points when adding or
subtracting decimal numbers then _____________ because _____________.