Disclaimer to Disclaimer: Dan does not regularly talk or write in the third person singular, it just ended up that way... that is all.
Occasionally, you know, when I'm sitting around figuring out how things work, I'll come some neat math guava moment. It's pretty cool and I hope you enjoy them. E-mail me at danprime@hotmail.com if you have any suggestions, spot errors or just plain feedback.
So for this month, this is a cool one that few people know and once you realize it you might slap your forehead and go "Dan that is so obvious!" if it is great! Then you should have no problem dividng numbers by 2. :) Ready or not here we go!
Foreword
You need to know how to divide numbers by 2. If you want to go fast, you should
be able to do this without a calculator. So like 14÷2=7, 39÷2=19.5
etc.
Onward
Well if you looked at the Foreword then you already have seen the trick!
If you want to multiply ANY number by 5 then divide that number by 2 and adjust.
For example: Say you want to muliply 14 by 5: 14 x 5. You could do it the
old way which would be: multiply 4 by 5, carry the 2 from the 20 over, multiply
1 by 5 plus 2 gives you 70
Wow that's a lot to do! Why not-
1)Divide 14 by 2 (giving you 7)
2)Adjust by moving the decimal (or comma if you're French) to the right!
3)7.0 becomes 70.0! (decimal or dot is added for example only)
Why it works Well look more closely at the steps, it's all about the
order of multiplication: You're diving by 2-Dividing by 2 is the same as multiply
by one half or better yet 0.5.
Notice the 5 in 0.5? To compensate the decimal displacement we multiply by
10. Since order doesn't matter we just leave that till later but in effect
multiply by 0.5 then by 10 is exactly the same as multiply by 5. Order does
not matter in multiplication.
Practice and Application
Check out a Flash visualization
and practice!
Your handy-dandy 9's multiplication
Please E-mail danprime@hotmail.com any questions, comments or things I left out, thanks!