please help. This is the problem What is 39 squared? :)

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- Dec 11th 2006, 01:07 PMAngel KingI'm not understanding this problem
please help. This is the problem What is 39 squared? :)

- Dec 11th 2006, 01:11 PMtopsquark
- Dec 11th 2006, 09:02 PMCaptainBlack
- Feb 1st 2007, 05:58 AMAlvinCY
There are many other ways to attack the problem, the box method which is long and tedious, simple multiplication, calculator... otherwise if you know the perfect square expansion:

39^2 = (40 - 1)^2... mainly cos 40 and 1 and nice numbers to work with.

= (40)^2 - 2*(40)*(-1) + 1^2

= 1600 + 80 + 1

= 1681 - Feb 1st 2007, 08:51 AMCaptainBlack
- Feb 1st 2007, 02:47 PMAlvinCY
Sorry. Change of signs... it was nearly 3 am wen I posted it, what I meant was in fact:

$\displaystyle 39^2$

$\displaystyle = (40 - 1)^2$

$\displaystyle = 40^2 - 2 \times 40 \times 1 + 1^2$

$\displaystyle = 1600 - 80 + 1$

$\displaystyle = 1521$

Which IS the correct answer