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
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