Algebra II For Dummies, 2nd Edition

Chapter 18

Ten Multiplication Tricks

IN THIS CHAPTER

Saving time on tests and homework problems with fast multiplication

Avoiding calculators and division (and sometimes multiplication itself)

You studied your multiplication tables way back in the third grade. You can quickly rattle off the product of 7 and 9 or 8 and 6 (can’t you?). In this chapter, you discover some new patterns and tricks to help you with products and processes — tricks that save you time. You’re also more apt to get the right answer when using these tricks (not that you ever have the wrong answer, of course). And think of how you can amaze your friends and colleagues by pulling seemingly tough answers out of thin air.

But before you get moving, I have a confession to make: I’ve snuck in an addition trick as well because I know that you’ll love it.

Squaring Numbers That End in 5

You know that 5 squared is 25. But what about , and so on? Don’t have your calculator on you? No worries. To square a number that ends in 5, follow these steps:

1. Write down the last digits of the answer: 25.

   The squared form of a number that ends in 5 always ends with 25.

2. Take the digit or digits in front of the original 5 and multiply ...