MULTIPLYING BY 2
Multiplying by 2 is another way of saying we are doubling a number or Simply that we are adding a number to itself. Doubling a number may be accomplished quickly without carrying by applying the following simple rule.
Rule:
( Starting from the first digit of the given number, double the digit if it is 4 or less and put the answer under the respective digits of the given number. For digits 5 to 9, subtract 5 and double the result. Place the answer under the respective digits of the given number. Now inspect the tentative answer. Each digit of the answer to the immediate left of a digit in the given number 5 or greater should be increased by 1. The result is the final answer. )
At first reading, this rule may sound more complicated than simply adding digit by digit. The beauty of this short-cut method is, however, that the answer is obtained immediately from left to right and we are never bothered by having to remember to carry over any digits. As an example, suppose we were asked to multiply 5,377 by 2. First let us write the given number, using our alphabetic identification:
A B C D
5 3 7 7
Starting from A, double each number less than 5 ( but not equal to 5 ); if the number is greater than 5, subtract 5 from it and double the result, placing a small line under each digit of the answer that is to the immediate left of a digit in the given number that is 5 or more. The reason for this small line will be explained shortly. In our given number, the first digit is 5; subtract 5 from this and double the result.
5 - 5 = 0; 0 + 0 + 0
Place 0 under the 5 and a small line under the space to the left of the 0 (since there is no number in that space). Our first result will look like this:
A B C D
5 3 7 7 Given number
-- 0 Tentative answer after first step
The next digit is less than 5, so we merely double it, and our answer begins to look like this now:
A B C D
5 3 7 7 Given number
--- 0 6 Tentative answer after second step
The C digit is a 7; subtract 5 from this and double the result.
7 - 5 = 2 ; 2 + 2 = 4
This is the C digit of the answer; but remember, a small line must be placed under the next digit to the left in the answer (the 6). We have now come this far in our answer:
A B C D
5 3 7 7 Given number
-- 0 6 4 Tentative answer after third step
Finally, the D digit is more than 5, so once again we obtain 4 and place a small line under the previous 4 in the answer. Our answer now looks like this:
A B C D
5 3 7 7 Given number
-- 0 6 4 4 Tentative answer after fourth step
Each underlined digit 18 increased by 1 to obtain the final answer.
10,754 /- Answer
No comments:
Post a Comment