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Difference of two squares and 39 × 41
By Kane Limbrick (P1960) on Sunday, January 23, 2000 - 06:49 pm:

I need to find a rule for questions such as
(2a +3) (2a -3)
&
(3b -7) (3b +7)
but I'm stuck. I think the rule helps you find the answer to questions such as 39 x 41 in your head quickly.

By Alastair Fletcher (Anf23) on Sunday, January 23, 2000 - 07:01 pm:

Hi Kane,

If you multiply out (a-b)(a+b) , you get a2 + a×b - b×a - b2 or just a2 - b2.

So to work out 39 × 41 quickly, put a=40 and b=1 and then 39 × 41 is 40×40 - 1 = 1599.

Alastair

By Kane Limbrick (P1960) on Sunday, January 23, 2000 - 09:20 pm:

Thank you very much! It took me a while to even understand your simple explanation but now I get it.

Thanks
KANE