# Teaching Math Tricks

Patrick Carter, Profession Speech Outline: Teaching, Some Math Tricks

OB 1) Math comes into play in life in many circumstances

A) When prepping for parties or events

B) When building something or working on a project

C) When calculating your budget

OB 2) Many of us are tied to our calculators

A) We normally have them & use them to make calc. easy & accurate

B) If we don't have one on hand we may be lost due to dependence

C) We still need to calculate things though Some Simple Tricks and Tips to Help with some of Life's Common Multiplication Needs

1) Multiplying a number up to 20 X 20 (In your head)

A) most of us know up to 10X10 pretty well, but what about 11 to 20?

B) can use a simple trick to easily do these numbers in your head

C) The Trick

- Take 15 x 13 for an example.
- Always place the larger number of the two on top in your mind.
- Then draw the shape of Africa mentally so it covers the 15 and the 3 from the 13 below. Those covered numbers are all you need.
*XXXXXXX BE**SURE TO DRAW THIS ON THE BOARD XXXX* - First add 15 + 3 = 18
- Add a zero behind it (multiply by 10) to get 180.
- Multiply the covered lower 3 x the single digit above it the "5" (3x5= 15)
- Add 180 + 15 = 195.
*XXXXXX HAVE US TRY IT NOW XXXXXXXX*

2) Square a 2 digit number ending in 5

The method: For this example we will use 25

- Take the "tens" part of the number (the 2 and add 1)=3
- Multiply the original "tens" part of the number by the new number (2x3)
- Take the result (2x3=6) and put 25 behind it. Result the answer 625.

75 squared ... = 7x8=56 ... put 25 behind it is 5625.

55 squared = 5x6=30 ... put 25 behind it ... is 3025.

*XXXXX THIS ONE SEEMS TO HAVE LESS USEFULNESS; IS THERE SOMETHING SPECIAL ABOUT 5* SQUARED NUMBERS?

3) The 11 Rule, Multiplying numbers by 11

A) we all know the 10 rule, add a zero onto the end, but 11 a bit more complicated

B) The method

- For this example we will use 54.
- Separate the two digits in you mind (5__4)
- Notice the hole between them!
- Add the 5 and the 4 together (5+4=9)
- Put the resulting 9 in the hole 594. That's it! 11 x 54=594

The only thing tricky to remember is that if the result of the addition is greater than 9, you only put the "ones" digit in the hole and carry the "tens" digit from the addition. For example 11 x 57 ... 5__7 ... 5+7=12 ... put the 2 in the hole and add the 1 from the 12 to the 5 in to get 6 for a result of 627 ... 11 x 57 = 627

Note from speaker: I'll have a handout for folks to work along with, as well as some questions to see if people can do the problem, if not the correct answer, I'll take the opportunity to work through the problem and see where the mistake was made to better explicate the method and remove any confusion.

*XXXXXXXXX have a problem or two for us to do after each math trick; about less tricks-time it with a few friends; when you have students do what you tell them to do, it is amazing how quickly the time gets used up. Trick 2 and 3 (especially 2) need more justification for their usefulness; I think if you explained a real life scenario for each one (not detailed-just real brief), it would help us see how the math would be useful. If you do have to get rid of a trick, make it 2.*

*Need a conclusion*