Today we talked about multiplying decimals. While a lot of kids remembered rules such as "you move the decimal in the answer as many places as there are digits behind decimals in the problem" our goal today was to justify WHY that is true. We talked about inverse properties- how multiplying and dividing by the same number leaves you with the same value- a great foundation for solving equations in Algebra! From there we made sure we don't ever say, "The decimal goes away" but justify it with- "I make 3.2 into a whole number by multiplying by 10" Then at the end we avoid, "I just count over one space and put the decimal" and say, "I use the inverse of multiplying by 10 and I divide by ten, which moves the decimal one place the the left." I think it helps to avoid what I call "Math-Magic"- when students make numbers/decimals/variables appear and disappear without justification, which can lead to errors in higher level math.
But... by the end, of the unit, students won't need to do every step we did today. Seeing patterns and using rules developed from these patterns is a crucial part of math learning. As we do more complicated decimals I'll be looking for those lightbulbs and students saying, "Oh, I can just place the decimal by...... this is justified by....."
Happy Math!!
But... by the end, of the unit, students won't need to do every step we did today. Seeing patterns and using rules developed from these patterns is a crucial part of math learning. As we do more complicated decimals I'll be looking for those lightbulbs and students saying, "Oh, I can just place the decimal by...... this is justified by....."
Happy Math!!