June 13th, 2007 @ 5:55 pm by: Marc
3 Fun Math Tricks
Here are 3 fun math tricks you can use to impress your geeky friends. All three tricks are based around the sequential manipulation of the numbers being used for input and output. One of my old college professors showed me these tricks a few years back. I thought it was pretty cool, so I figured I’d share it. Enjoy yourselves!
1 Through 9 to 9 Through 1:
1 x 8 + 1 = 9
12 x 8 + 2 = 98
123 x 8 + 3 = 987
1234 x 8 + 4 = 9876
12345 x 8 + 5 = 98765
123456 x 8 + 6 = 987654
1234567 x 8 + 7 = 9876543
12345678 x 8 + 8 = 98765432
123456789 x 8 + 9 = 987654321
Sequential Input to All 1’s:
1 x 9 + 2 = 11
12 x 9 + 3 = 111
123 x 9 + 4 = 1111
1234 x 9 + 5 = 11111
12345 x 9 + 6 = 111111
123456 x 9 + 7 = 1111111
1234567 x 9 + 8 = 11111111
12345678 x 9 + 9 = 111111111
123456789 x 9 + 10 = 1111111111
All 1’s to a Numeric Palindrome:
1 x 1 = 1
11 x 11 = 121
111 x 111 = 12321
1111 x 1111 = 1234321
11111 x 11111 = 123454321
111111 x 111111 = 12345654321
1111111 x 1111111 = 1234567654321
11111111 x 11111111 = 123456787654321
111111111 x 111111111 = 12345678987654321
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5 Comments
July 27th, 2007 at 6:11 am
There’s also that old trick (notice there’s no “8″):
12345679 X 9 = 111111111
so
12345679 X 18 = 222222222
12345679 X 27 = 333333333
…
August 8th, 2007 at 7:57 pm
Don’t forget that 643×192=123456
or that 1/8.1=.1234567
May 4th, 2008 at 6:45 pm
and that 2/3=.666
(someone ever heard of the 23 enigma????)
May 6th, 2008 at 11:12 pm
even better, Shiny Freak is :
2^2 +3^2+5^2 +7^2 +11^2 +13^2+17^2=666
Uparrow means to the power of.
Note that (not counting 1, which today is considered a prime) this is the sum of the squares of the first seven
primes.
Without a telescope, there were seven planets:
back in the day that was: Sun, Moon, Mercury,
Venus, Mars, Jupiter and Saturn.
Which is to say: The true origin of the mystic number 666 is in the numerology cult of Pythagorus,
which predates the Christian Bible.
It is not in greek gematria. And has nothing to do with beasts or devils or antichrists.
p.s. two thirds is NOT = .666
It equals .6666…..
May 6th, 2008 at 11:26 pm
Based on the same ideas: Someone hits all fives on their
( say ten place) calculator, and you tell them the square root of that number with many correct
significant figures before they can punch the square root key.
To approximate the square root of 5555555555, you can note that 1/9= .111111111…. and thus 5/9=
.555555555…..
Thus, up to small error and a shift of the decimal point
we know that the square root in question is equal
to one third of the square roof of 5.
Now, by standard elementary school methods, or by using a memorized log table, or by imagining a slide rule, you know the square root of five to a bunch of decimal places.
Thus, you can quite a few places in your head, quite quickly.
Used to do this sort of thing for entertainment when I was a child.
p.s.
When you use a slide rule a lot, it becomes pretty easy to see one in your head. And if you took
trig before calculators, there was a lot of solving triangles using nasty arithmetic ( law of sines, tangents etc.) that was done with log tables. After a few weeks, you basically knew the log table (to three
places) by heart, and simple interpolation ( which you did constantly) gave you four places. Or, you could notice that the five place log table was easy to memorize ( though large) by starting with a memorized three place log table.
So, you could do slide rule type stuff in your head.
The absolutely best book on mental calculation
, in a class of its own, is titled ” Dead Reckoning”.
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