For the 1st time, I'll tell You about how Multiplying numbers upt to 10. I know you do not need to use this method in the calculation using only 2 digits, but it is important to study because it is the basis of the method itself.
Let's learn it!
We will begin by learning how to multiply numbers up to 10 x 10.
This is how it works:
We'll take 7 x 8 as an example.
Write 7 x 8 = down on a piece of paper and draw a circle below each number to be multiplied.
Now go to the first number to be multiplied, 7. How many more do you need to make 10? The answer is 3.
Write 3 in the circle below the 7. Now go to the 8. What do we write in the circle below the 8? How many more to make 10? The answer is 2. Write 2 in the circle below the 8.
Your work should look like this:
Now subtract diagonally. Take either one of the circle numbers (3 or 2) away from the number, not directly above, but diagonally above, or crossways. In the other words, You either take 3 from 8 or 2 from 7. You only subtract one time, so choose the subtraction you find easier. Either way, the answer is the same, 5. This is the first digit of your answer.
8 - 3 = 5 or 7 - 2 = 5
Now multiply the numbers in the circles. Three times 2 is 6. This is the last digit of your answer. The answer is 56. This is how the completed problems look.
If you know the 2 timestable reasonably well, You can easily master the tables up to the 10 times table, and beyond. Let's try another example, 8 x 9.
How many more to to make 10? The answer is 2 and 1. We write 2 and 1 in the circles below the numbers. What do we do now? We subtract diagonally.
8 - 1 = 7 or 9 - 2 = 7
Seven is the first digit of your answer. Write it down. Now multiply the two circled numbers
2 x 2 =1
Two is the last digit of the answer. The answer is 72.
Isn't that easy? I'll tell You more in the next post. :)
Source: Speed Mathematics (secret skills for quick calculation) by Bill Handley
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