Additional Math Pages & Resources

Wednesday, December 2, 2009

Take away for six at seven?

English and Australian readers know that when I say Take Away,  I mean Food To Go.  I am asking about dinner for six people at seven pm.

In America, Take Away has many different meanings.

To deduct, remove, carry off, withdraw, to subtract. To divest, shuck, unwrap, strip, shell, scalp, cross off, strike out, delete, erase, weed out, detract, reduce, trim. Confiscate. Withhold.

As in T.A. your driving license, T.A. your X-Box privileges, etc.

I could go on and on. I found a site dedicated to improving the takeaway in my golf swing (takeaway in golf is when you pull the club back/up before swinging forward/down to hit the ball).

In math it means subtraction. The fourth of our primary operations. The inverse of addition.

So mathematically I could say seven take away six leaves one. No food involved!

There are a few special terms to learn for subtraction - the minuend (what you start with), the subtrahend (what you take away) and the difference (what's left). The special symbol is "-".

Subtraction eventually forces us to learn about borrowing (carrying, trading, regrouping).

I bought a bunch of stuff at the store.  I subtracted what I spent from the balance in my checking account. My balance is now less than zero. I have to borrow money to carry me over until next month.

That's not the kind of borrowing I was talking about (but it's a common result of subtracting).  I meant this kind:

Nowadays we usually say regrouping rather than borrowing. We take a group of ten ones from the tens column, add them to our existing, too-small group of ones, and then we can subtract. Like this, where we regroup the one ten so we will have eleven ones, from which we take eight:

Subtraction doesn't have all of the same special properties that addition does.
  • Commutative - does changing the order of the numbers alter the difference? 2-3 ≠ 3-2. Subtraction is not commutative because changing the order of the numbers does change the answer.
  • Associative - when three or more numbers are subtracted, does regrouping them alter the difference? (5-1)-4 ≠ 5-(1-4). Subtraction is not associative because regrouping the numbers does change the answer.

I think that's quite enough of these basic math operations, don't you? But wait, one more thing...

I don't know about your family, but if we get some take away, and try to take away some of the other person's fries, we always get caught. Notice that hand sneaking in there behind my fish and chips?

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