Callistonian.net

Poetic forms like imabic trimeter are barely comprehensible to me. In school, whenever I was interrogated about a poem’s meter, my thoughts were, “Uh? I don’t really know; I hate this.” Luckily, some mathematical things, like the cubed roots of certain numbers, are much easier to figure out. Trust me!

Let’s say you have a number like 157464. It’s cubed root is 54 (157464 is 54 x 54 x 54, which is also know as 54^3). How long does it take to figure out that 157464’s cubed root is 54? Less time than it takes to figure out if a line of poetry is in imabic trimeter or pentameter or what have you. How much cleverer does a girl who can take the cubed root of 157464 in 5 seconds look than the girl who can tell you about a poem’s meter in 5 seconds? Way cleverer, you know.

Honestly, if the number being cubed is two-digits, it can be easily discovered without a calculator. But first you must know a few things.

1. The numbers 1-9 cubed (knowing this is sort of necessary)
   • 1 cubed is 1
   • 2 cubed is 8
   • 3 cubed is 27
   • 4 cubed is 64
   • 5 cubed is 125
   • 6 cubed is 216
   • 7 cubed is 343
   • 8 cubed is 512
   • 9 cubed is 729
2. The last digit of the cube of those numbers (the bold numbers)¹

That’s it. Now, this is how it works—Let’s say you have a number like, oh, 300763. First, ignore its last three digits. Mentally, 300763 should become 300. 300 is greater than 216 (6 cubed) but less than 343 (7 cubed), so the cubed root of 300763 is going to be 60-something. Second, think of 300763’s last number. It’s 3, which is the same number as the last digit of 7 cubed (343), so the cubed root of 300763 is 67.

Another one—21952.

1. 21952 (drop the last 3 digits)
2. 21 is greater than 2 cubed (8) but less than 3 cubed (27), so its cubed root must be 20-something.
3. 21952 ends in a 2. 8 cubed also ends in a 2. Thus, 21952’s cubed root must be 28.

That’s all there is to it. Now it’s time to shock your friends, parents, older siblings, lovers, and enemies. Tell them to grab a calculator and start cubing 2-digit numbers. Watch their mouths hang open in utter amazement as you tell them the numbers they’ve cubed. :D lulz.

Back story: One of the teachers was doing this at work today… I half-learned the trick from him.

¹ The last digits of 1-9 cubed are easiest to remember if they (1-9) are pictured on a line. The numbers on the end 1 & 9 are themselves. The next few numbers (2, 3, 8, 7) are 10 minus themselves, while the ones in the middle (4, 5, 6) are also themselves.
² There’s more on the subject here.