Multiplying numbers?

[yhack]

Lol, my old maths teacher used to call it the 'Harry Potter Method' because you go diagonally

 

[DaveF]

not very easy to do in the head.

If you want to do the calculation in your head, and (a) you can write the answer down, and (b) starting writing at the right is OK, the easiest way is to do pair off so that first you do the units, then the 10s, 100s, etc.

i.e. 123 x 456 = 3x6 + (2x6+3x5)x10 + (1x6+2x5+3x4)x100+(1x5+2x4)x1000 + (1x4)x10000
= 18 + 27 x 10 + 28 x 100 + 13 x 1000 + 4 x 10000
= 56088

How you'd actually do this in your head is (where everything written down will end up in reverse order)

3x6 = 18. Write down 8, carry the 1 (in your head).
1 + 2x6 = 13. 13 + 3x5 = 28. Write down the 8, carry the 2.
2 + 1x6 = 8. 8 + 2x5 = 18. 18+3x4 = 30. Write down the 0, carry the 3
3 + 1x5 = 8. 8 + 2x4 = 16. Write down the 6, carry the 1.
1 + 1x4 = 5. Write down the 5, and you're done.

 

[[TR]SweetSpotOz]

If you want to do the calculation in your head, and (a) you can write the answer down, and (b) starting writing at the right is OK, the easiest way is to do pair off so that first you do the units, then the 10s, 100s, etc.

i.e. 123 x 456 = 3x6 + (2x6+3x5)x10 + (1x6+2x5+3x4)x100+(1x5+2x4)x1000 + (1x4)x10000
= 18 + 27 x 10 + 28 x 100 + 13 x 1000 + 4 x 10000
= 56088

How you'd actually do this in your head is (where everything written down will end up in reverse order)

3x6 = 18. Write down 8, carry the 1 (in your head).
1 + 2x6 = 13. 13 + 3x5 = 28. Write down the 8, carry the 2.
2 + 1x6 = 8. 8 + 2x5 = 18. 18+3x4 = 30. Write down the 0, carry the 3
3 + 1x5 = 8. 8 + 2x4 = 16. Write down the 6, carry the 1.
1 + 1x4 = 5. Write down the 5, and you're done.

Hmm. Don't mean any offense but this seems like a crazy complicated way of doing it!

 

[manoz]

Diagonals are because you add each diagonal to get the product.

Gots it, I was tryin to do it like DaveFs example,

 

[DaveF]

Hmm. Don't mean any offense but this seems like a crazy complicated way of doing it!

There's a trade off - it's might seem a little more complex, but ordering the calculation as I describe results in something most people find much easier to do mentally. And the complexity is more in the description that the actuality. Each individual operation is simple, and the most complicated it ever gets is having to add a 2 digit number to a 3 digit one. The main advantage is that that's basically all you have to remember during the calculation - for most people the limiting factor of doing these things in their head isn't "how complicated the sums are", it's simply remembering all the intermediate totals in their head.

 

[Solari]

I must admit I use the grid to do long multiplication... it seems childish but works.

As for long division, I can't remember how to do it *shame*

It's shocking to think that I passed HNC level Calculus about ten years ago... Must be the result of beer, women and other extra-curricular activities Thankfully the most challenging my job gets mathematically is percentages, number base conversions and other mundane calculations!

 

[Vertigo]

I'm very good at identifying whether a number is prime or not just by looking at it.

Might come in handy if you ever find yourself in a strange matrix of interconnecting rooms full of booby traps

 

[Chaos]

Get a book on the Trachtenberg Speed System.

 

[Vonhelmet]

Might come in handy if you ever find yourself in a strange matrix of interconnecting rooms full of booby traps

Aha! True! Not actually seen that film. Don't they turn out to be a particular variety of primes, like happy primes or something?