Associated Topics || Dr. Math Home || Search Dr. Math

### Multiplication Trick

```
Date: 02/08/98 at 21:49:38
From: Rosemarie Lee
Subject: Why is the answer right?

I am a fourth grader and I have been doing multiplication and
division. While playing with multplication I discovered that when I
multiply 25x25 or 35x35 or 45x45 all the way up to 95x95 I can do the
answer in my head by multiplying the first digit times the next
highest number - like in 25 x 25 I go 2x3 is 6 and 25x 25 is 625; or
45x 45 is 4x5 is 20 so the answer is 2025.

I am thinking about three-digit numbers like 225x225 because I think
they should work too. I need to think about it some more. Does this
work with numbers ending in 25 because 25 multiplies itself, or why
does it work?

I did my answers on a calculator and I know they are right. I would
like to know. Can you help me find out?

Thanks you for helping me.
Douglas
```

```
Date: 02/10/98 at 11:15:00
From: Doctor Pete
Subject: Re: Why is the answer right?

Hi,

First of all, let me say that you're very observant! What you're
saying is quite true: For example,

35x35 = 1225.

But strictly speaking, this is because

35x35 = (40 - 5) x (30 + 5)
= 40 x (30 + 5) - 5 x (30 + 5)
= (40x30 + 40x5) - (30x5 + 25)
= (4x3)x100 + (40-30)x5 - 25
= 12x100 + 50 - 25
= 12x100 + 25
= 1225.

And so you can see how this might be generalized to the square of any
number that ends in 5; for example,

115x115 = (11x12)x100 + 25 = 13225.

For example, let's pick the number

3251165

and see what its square might be. We have (skipping a few steps)

3251165x3251165 = (3251170 - 5) x (3251160 + 5)
= (325117x325116)x100 + (3251170 - 3251160)x5 - 25
= (325117x325116)x100 + 10x5 - 25
= (325117x325116)x100 + 25.

(Now, I'm going to cheat a bit and just say that 325117x325116 =
105700738572, since the point isn't really about finding the square,
but that you could pick any number and see that it still works.) So we
see that we are always left with some multiple of 100, times 25. You
chop off the units' 5 of the original number, and take product of this
number with the number 1 greater.  When you become familiar with
algebra, it will become more clear why this is always the case.

-Doctor Pete,  The Math Forum
Check out our web site!  http://mathforum.org/dr.math/
```
Associated Topics:
Elementary Multiplication

Search the Dr. Math Library:

 Find items containing (put spaces between keywords):   Click only once for faster results: [ Choose "whole words" when searching for a word like age.] all keywords, in any order at least one, that exact phrase parts of words whole words

Submit your own question to Dr. Math
Math Forum Home || Math Library || Quick Reference || Math Forum Search