## Tuesday, July 25, 2006

### Finger Arithmetic Part Nine - Subtraction

Continuing finger arithmetic from Part Eight. This one is called Part Nine. It is the tenth part in the series. It introduces subtraction.

Subtraction is the opposite of addition. It's sort of addition backwards. You might be able to figure it out, using what you've learned from addition. Indeed, i was unable to teach my eight year old subtraction. I announced that i would teach him, and he just went ahead and did it. Now, if he'd done it wrong, i'd have corrected him. And when he makes mistakes, i walk him through the steps. But he really seemed to go through all nine lessons in reverse, taking about ten seconds. I don't hold him back. The correct direction is forward to the goal.

In lesson zero, we had counting. Count from nine to zero. It was then pointed out that counting was adding one. Counting down is the same as subtracting one. Remember to say the answers out loud. Try counting from twenty to zero.

Now we'll subtract small numbers. Consider 4 - 3. Set four fingers and subtract three fingers. Read the result: One. This covers 1 - 1, 2 - 1, 3 - 1, 4 - 1, 2 - 2, 3 - 2, 4 - 2, 3 - 3, 4 - 3, 4 - 4. Practice these.

Some big numbers with small numbers are easy. Consider 9 - 3. Set nine. Take away three fingers. Read the answer: six. This covers 9 - 1, 9 - 2, 9 - 3, 9 - 4, 8 - 1, 8 - 2, 8 - 3, 7 - 1, 7 - 2, 6 - 1.

Next is a half carry, using Five's Compliments. Recall that the Five's Compliment of 1 is 4, and 4 is 1, and 2 is 3 and 3 is 2. Consider 7 - 3. Set seven. Note that there aren't three fingers to subtract. We're going perform this by observing that 7 - 3 is the same as 7 + 2 - 5. Remember that the Five's Compliment of three is two, so you add two and subtract five. Add two fingers. Subtract the thumb. Read the answer: four.

Subtracting five is easy. Consider 7 - 5. Set seven. Take away the thumb. Read the answer: two. This covers 9 - 5, 8 - 5, 7 - 5, 6 - 5, and 5 - 5.

Subtracting bigger numbers. Consider 9 - 7. Set nine. Seven is five plus two - thumb and two fingers. You have two fingers to take away, so do it. You have a thumb to take away, so do it. Read the answer: two. This covers 9 - 9, 9 - 8, 9 - 7, 9 - 6, 8 - 8, 8 - 7, 8 - 6, 7 - 7, 7 - 6, 6 -6. Need you be reminded to practice?

Next lesson: two digit subtraction. Bring your thinking caps.

Again, you are referred to the example generator, which can provide you with an infinite number of examples. Each time you click here, you get a new page. Use your back button to get back to the lesson. This link provides two digit examples. As this lesson hasn't taught two digit subtraction, feel free to ignore any problems involving two digits anywhere.

You might just want to go to the web site, so you can ask the generator yourself. Addition, subtraction or mixed problems, how many digits, more than just two numbers per problem? Answers on the same page or another page? Maybe you want one hundred pages to get you through vacation.