Tuesday, June 12, 2018
Our fun with 5s doesn't end there. We talked about how to square numbers in your head before, but it turns out that things get a whole lot easier when squaring a two-digit number that ends in 5. Here's the trick: Any time you square a two-digit number that ends in 5, the last digits of the answer will be 25 and the digits before that are given by multiplying the first digit of the number by the number that's one greater.
The third trick for today has to do with multiplying any number by 9, 99, 999, or any other number that's 1 less than a power of 10. What makes all of these wild 9 numbers special? In a problem like 44 x 9, the trick is to recognize that 44 x 9 = 44 x (10 - 1). The distributive property of multiplication tells us that this is the same as 44 x 10 - 44. And since it's easy to multiply by a power of 10, looking at the problem this way makes it much easier to solve. In particular, it tells us that 44 x 9 = 44 x 10 - 44 = 440 - 44 = 440 - 40 - 4 = 396 (sharp-eyed math fans may notice a trick there related to Math
You can use today's fourth tip any time you're multiplying one number by another number that's a power of 2. Which means that any time you're multiplying some number by 2, 4, 8, 16, 32, 64, and so on, this is your ticket to mental math bliss. Instead of going through the usual multiplication process, in this case all you have to do is double the number you're multiplying for each power of 2 in the other number.
The previous trick is really just a special case of today's fifth and final (and I think coolest) trick that you can use whenever one of the numbers you're multiplying is even. Let's say you're multiplying 47 x 24. Since 24 is an even number, let's use the idea of doubling and halving to solve this problem quickly.
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