Mathemagics: How to Look Like a Genius Without Really Trying by Arthur Benjamin, Michael Brant Shermer

By Arthur Benjamin, Michael Brant Shermer

Utilizing confirmed strategies, this quantity exhibits find out how to upload, subtract, multiply and divide swifter than is feasible with a calculator or pencil and paper, and is helping readers triumph over their anxiousness approximately math.

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Let's tackle our first problem: 42 X 7 First, multiply 40 x 7 = 280. Next, multiply 2 x 7 = 14. Add 14 to 280 (left to right, of course) to arrive at 294, the correct answer. We illustrate this procedure below. We have omitted diagramming the mental addition of 280 + 14, since you learned in Chapter 1 how to do this computation. 42 (40 X 40x7= 2 X 7 = + 2) 7 280 + 14 294 At first you will need to look down at the problem while calculating it to recall the next operation. With practice you will be able to forgo this step and compute the whole thing in your mind.

Rounding Up You saw in the last chapter how useful rounding up can be when it comes to subtraction. The same goes for multiplication, especially when the numbers you are multiplying end in an 8 or 9. Let's take the problem of 69 x 6, illustrated below. On the left we have calculated it the usual way, by adding. On the right, however, we have rounded 69 up to 70, which is an easy number to multiply. And for many people, it is easier to subtract 420 - 6 than it is to add 360 + 54 when calculating mentally.

All you have to add is the square of 1 (the number by which you went up and down from 41), giving you 1680 + 1 = 1681. Can squaring a 2-digit number be this easy? Yes, with this method and a little practice, it can. And it works whether you initially round down or round up. For example, let's examine 77 2 , working it out both by rounding up and by rounding down: 80 84 +7/ +3/ 5880 / 70 + 49 (72) 5929 or n2 5920 -3"'- / 74 + 9 5929 (32) 32 MATHEMAGICS In this instance the advantage of rounding up is that you are virtually done as soon as you have completed the multiplication problem because it is simple to add 9 to a number ending in O!

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