Nearly Squares 

Problems:

You know that            10 x 10   ( or 102 )  = 100

What’s                        9  x 11 ?

Can you see the connection?!

 

Try       19 x 21  (close to 20 x 20),              49 x 51           and      99 x 101        

as well, to check your hypothesis.

 

Can you see how the diagram below explains why the method works?

 

So, for example,     9 x 11 = 102 - 12 

Now calculate 18 x 22, and see how much less this is than 20 x 20

 

Try       38 x 42,          and      98 x 102                     as well, to check your method.

This method can be used for any square number that you know the answer to, and the related multiplications either side of it.

For example, 37 x 43 = 402 - 32 = 1600 – 9 = 1591

 Use the same approach to work out, quickly:

 47 x 53           94 x 106         76 x 84           112 x 128      5.8 x 6.2

Open the File as a Word Document

 

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Last modified: June 18, 2007