Mental Math with Negatives

Mental Math · Axiom Academy

LESSON Mental Math with Negatives When the number you take away is bigger, find the size of the shortfall — then attach the minus sign. 1. Zero Is Not the End of the Line Counting seems to stop at zero only because that is where we usually stop drawing. The line keeps going. A negative number is not "less than nothing" — it is a position , and you read it in two pieces. The numeral tells you how far you are from zero; the sign tells you which side you are on. So -5 sits exactly five steps from zero, on the left-hand side, the mirror of where 5 sits. Same distance from zero, opposite directions 2. Subtracting Straight Past Zero Now watch 23 - 58 happen. Stand on 23 and start taking away. The first 23 you take brings you down to zero exactly — but you are not finished, because you were asked to take away 58, not 23. There are still 58 - 23 = 35 left to take, and taking them carries you straight through zero onto the minus side. You come to rest 35 steps to the left of zero: -35 . The same crossing, with a bank account You have 23 dollars in an account and a 58-dollar charge lands on it. The balance does not stop at empty: the first 23 dollars of the charge clear out the account, and the remaining 35 push it into the red. Your new balance is 35 dollars overdrawn — a balance of -35 . 3. Find the Size, Then Attach the Sign

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