Sunday, October 18, 2009

Math Multiplication Tricks

A number of multiplication tricks can make multiplication fun and simple. In fact, these tricks make it possible to calculate equations in your head. It is simply a matter of practice.

To multiply squares of five, write down 25. All squares of five end in 25. Next, add one to the first digit and multiply. For example, 45x45=2025. We first write down 25. We then add 1 to the first digit which is 4. This gives us 5. We multiply 5x4 to get 20. We write 20 next to 25 to get the answer 2025.

Now we will do 85x85. We write down 25. We add 1 to 8 which is 9. We multiply 9 by 8 to get 72 and write it down to get 7225. 85x85=7225.

For squares of 90 to 99, we have a simple formula. Subtract the number needed to reach 100 from the square. For example, 98 is two from 100, so we subtract two to get 96. We write this number down. Next we square this number and write it down. 2x2=4, so we write down 4 but we need two digits to the right of 96, so we write 9604. 98x98 = 9604.

Now we will try 96x96. 96 is four from 100, so we write 92. Next we square four. 4x4=16, so we write 9216. 96x96=9216.

Unfortunately, not all squares are quite so simple. With 27x27, we need to use the closest multiple of ten to calculate the answer. The closest multiple of ten is thirty. It is three away, so we subtract three from 27 to get 24. Now we must multiply 24 by 30. This is 720. Now we square three to get nine and add this number. 27x27=729.

Now we can do 49x49. 49 is one from fifty, so we get 48. We multiply 48 by 50 to get 2400. Next we square one to get one. 49x49=2401.

With multiplication problems up to 20x20 we have a simple formula. We can use 16x13 as an example. Always start with the higher number because this makes the calculation easier to perform. Remove the first digit from 13 and this gives us 3. Now add 16+3. This gives us 19. Now add a 0 to 19 to get 190. Finally multiply the last digits of each number. We multiply 6x3 to get 18. We add 18 to 190 to get 208. 16x13=208.

Now we can do 19x14. We add 19+4 to get 23. Add a 0 to get 230. Now we multiply 9x4 to get 36 and add this number to 230. We get 266. 19x14=266.

With multiplication by 11, we also have a simple formula. If we want to calculate 54x11, we write down the 5 and the 4 with a space between them. We can write 5_4. Next we write the sum of 5 and 4 in this space. 5+4=9, so we write 594. 54x11= 594.

Now we can solve 72x11. We write 7_2 so that we have the 7 and 2 with a space between them. The sum of 7 and 2 is 9, so we write this in the space. 72x11=792.

If the sum if greater than ten, however, we write down the ones digit and carry over the tens digit. I can demonstrate with 67x11. We write 6_7. 6+7=13, so we write the ones digit in the space. This gives us 637 but we must add 1 to 6 to get the correct answer, 737. 67x11=737.

Multiplication tricks can make multiplication not only more enjoyable but possible to calculate with only your brain. It is simply a matter of practice. I wish that I had known these tricks when I was in school.


Maggie said...

Take some math courses then! I am working on a second degree in Sci and am taking Math! Thanks for the tips!!!

Les Zsoldos said...

I like math but I think I have to focus on Korean now.

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