3 years ago, I posted a tutorial about estimating square roots of non-perfect squares, including tips and tricks.

Since then, I've wondered if there was a general formula for estimating other roots, such as cube roots, fourth roots, and so on. Reddit user InveighsiveAd informed me that there's a simple general formula very similar to the method I've taught for square roots! Once you pick up the basic idea of this method, you'll be able to astound friends, family, and teachers.

The approach for estimating roots originates from an approach developed by Leonhard Euler, and involves taking derivatives, so I won't delve into the math behind why this works here. I'll focus more on the resulting formulas, which can be used to

The method I taught for estimating square roots basically boiled down to this formula, where a was a perfect square equal to or less than x, and b was equal to x - a:

With Euler's method, we'll be estimating roots using the same basic approach of breaking up a number into a number which is a perfect power (square, cube, 4th power, etc.) and the difference between that power and the targeted number. The following formula may look scary at first, but it's simpler than it looks:

y is simply the root we wish to know. For square roots, y would equal 2, for cube roots, y would equal 3, and for 4th roots, y would equal 4. As a matter of fact, I'm not going to concern this article with anything past 4th roots, as this quickly becomes complex. Here are the formulas for square, cube and 4th roots individually:

These look worse than they really are. Remember that a is always chosen to be a perfect power, so you're working with an easily determined number. If you were going through this process for cube root, and using 729 for a, the cube root of 729 would be 9. So, any where you see the cube root of a, you can mentally replace it with 9, in this example.

Obviously, knowing perfect squares up through 31 will be of help, as in the original method. Knowing the perfect cubes from 1 to 10, as many Grey Matters readers already do, will allow you to estimate cubes of number up to 1,000. Memorizing or being able to quickly calculate perfect 4th powers will allow you to estimate 4th powers up to 10,000!

For those confused by the ± symbol in the equations, it simply means that we're going to choose a to be the closest perfect power, and adjust b accordingly. For example, if we want the cube root of 340, then we'd use 343 (7^{3}), and work it out as the cube root of 340 as the cube root of (343 - 3).

Let's estimate the cube root of 340 as a full example. As explained above, we've already broken this up into the cube root of (343 - 3). Your mental process might go something like this:

How close is 6^{48}⁄_{49} to the cube root of 340? The two numbers are *very* close, as this Wolfram|Alpha comparison shows!

Colin Beveridge, of Flying Colours Maths has helpfully pointed out that the error in the method will increase as you get approach the geometric mean of two closest consecutive perfect powers. For example, when using this method to find the cube root of 612, which is close to 611 (the approximate geometric mean of 512 and 729), you'll be farther off.

Let's find out exactly how far off we would be. The cube root of 612 could be worked out as (729 - 117), but (512 + 100) is closer, so we'll use the latter. Working this out, we'd get:

Wolfram|Alpha shows that 8^{25}⁄_{48} ≈ 8.52, while the actual cube root of 612 ≈ 8.49. It's off by about 3 hundredths, but that's still a good estimate!

As an added bonus, if you wind up with a fraction whose denominator ends in 1, 3, 5, or 7, you can use the techniques taught in *Leapfrog Division* or Leapfrog Division II to present your estimate with decimal accuracy! Yes, it's just the same number presented differently, but working out decimal places in your head always comes across as impressive. Personally, I reserve the decimal precision for when I know the root is close to a perfect power.

Try this approach out for yourself. If you have any questions, feel free to ask them in the comments!

## Estimating Roots

Published on Sunday, March 29, 2015 in fun, math, mental math, self improvement

## Grey Matters' 10th Blogiversary!

Published on Saturday, March 14, 2015 in books, fun, magic squares, Martin Gardner, math, memory, memory feats, mental math, nim, Pi, puzzles, reviews, self improvement, site features

Ever since I started this blog, I've been waiting for this day. I started Grey Matters on 3/14/05, specifically with the goal of having its 10th blogiversary on the ultimate Pi Day: 3/14/15!

Yes, it's also Einstein's birthday, but since it's a special blogiversary for me, this post will be all about my favorite posts from over the past 10 years. Quick side note: This also happens to be my 1,000th published post on the Grey Matters blog!

Keep in mind that the web is always changing, so if you go back and find a link that no longer works, you might be able to find it by either searching for a new place, or at least copying the link and finding whether it's archived over at The Wayback Machine.

## 2005

My most read posts in 2005 were*25 Years of Rubik's Cube*(at #2), and

*Free Software for Memory Training*(at #1). It was here I started to get an idea of what people would want from a blog about memory feats.

## 2006

In the first full January to December year of Grey Matters, reviews seemed to be the big thing. My reviews of*Mathematical Wizardry*,

*Secrets of Mental Math*, and

*Mind Performance Hacks*all grabbed the top spots.

## 2007

This year, I began connecting my posts with the interest of the reader, and it worked well. My series of “Visualizing” posts,*Visualizing Pi*,

*Visualizing Math*, and

*Visualizing Scale*were the biggest collectively-read posts of the year.

Fun and free mental improvement posts also proved popular in 2007.

*Unusual Lists to Memorize*, my introduction to

*The Prisoner's Dilemma*, and my look at

*Calculators: Past, Present, and Future*(consider Wolfram|Alpha was still 2 years away) were well received!

*10 Online Memory Tools...For Free!*back-to-back with my

*Memorizing Poetry*post also caught plenty of attention.

## 2008

I gave an extra nod to Pi this year, on the day when Grey Matters turned Pi years old on May 5th. The most popular feature of the year was my regularly update list of*How Many Xs Can You Name in Y Minutes?*quizzes, which I had to stop updating.

Lists did seem to be the big thing that year, with free flashcard programs, memorizing the elements, and tools for memorizing playing card decks grabbed much of the attention in 2008.

## 2009

Techniques took precedence over lists this year, although my series on memorizing the amendments of the US Constitution (Part I, Part II, Part III) was still popular. My web app for memorizing poetry, Verbatim, first appeared (it's since been updated). Among other techniques that caught many eyes were memorizing basic blackjack strategy, the*Gilbreath Principle*, and

*Mental Division with Decimal Precision*.

## 2010

This year opened with the sad news of the passing of Kim Peek, the original inspiration for the movie*Rain Main*. On a more positive note, my posts about the game Nim, which developed into a longer running series than even I expected, started its run.

As a matter of fact, magic tricks, such as

*Bob Hummer's 3-Object Divination*, and puzzles, such as the 15 Puzzle and Instant Insanity, were the hot posts this year.

Besides Kim Peek, 2010 also saw the passing of Martin Gardner and BenoĆ®t Mandelbrot, both giants in mathematics.

## 2011

The current design you see didn't make its first appearance until 2011. Not only was the blog itself redesigned, the current structure, with Mental Gym, the Presentation section, the Videos section, and the Grey Matters Store, was added. This seemed to be a smart move, as Grey Matters begin to attract more people than ever before.The new additions to each section that year drew plenty of attention, but the blog has its own moments, as well. My list of

*7 Online Puzzle Sites*, my update to the Verbatim web app, and the

*Wolfram|Alpha Trick*and

*Wolfram|Alpha Factorial Trick*proved most popular in 2011.

My own personal favorite series of posts in 2011, however, was the

*Iteration, Feedback, and Change*series of posts:

*Artificial Life*,

*Real Life*,

*Prisoner's Dilemma*,

*Fractals*, and

*Chaos Theory*. These posts really gave me the chance to think about an analyze some of the disparate concepts I'd learned over the years when dealing with various math concepts.

## 2012

In 2012, I developed somewhat of a fascination with Wolfram|Alpha, as its features and strength really began to develop. I kicked the year off with a devilish 15-style calendar puzzle, which requires knowing both how to solve the 15 puzzle and how to work out the day of the week for any date in your head! Yeah, I'm mean like that. I did, however, release*Day One*, my own original approach to simplifying the day of the week for any date feat.

*Estimating Square Roots*, along with the associated tips and tricks was the big feat that year. The bizarre combination of controversy over a claim in a Scam School episode about a 2-card bet and my approach to hiding short messages in an equation and Robert Neale's genius were also widely read.

## 2013

After we lost Neil Armstrong in 2012, I was inspired to add the new*Moon Phase For Any Date*tutorial to the Mental Gym. A completely different type of nostalgia, though, drove me to post about how to program mazes. Admittedly, this was a weird way to kick off 2013.

Posts about the Last Digit Trick,

*John Conway's Rational Tangles*, and Mel Stover were the first half of 2013's biggest hits on Grey Matters.

I also took the unusual approach of teaching Grey Matters readers certain math shortcuts without initially revealing

*WHY*I was teaching these shortcuts. First, I taught a weird way of multiplying by 63, then a weird way of multiplying by 72, finally revealing the mystery skill in the 3rd part of the series.

## 2014

Memory posts were still around, but mental math posts began taking over in 2014. A card trick classically known as Mutus Nomen Dedit Cocis proved to have several fans. The math posts on exponents, the nature of the Mandelbrot set, and the Soma cube were the stars of 2014. Together, the posts*Calculate Powers of e In Your Head!*and

*Calculate Powers of π In Your Head!*also grabbed plenty of attention.

## Wrap-up

With 999 posts before this one, this barely even scratches the surface of what's available at this blog, so if you'd made it this far, I encourage you to explore on your own. If you find some of your own favorites, I'd love to hear what you enjoyed at this blog over the years in the comments below!## Estimating Compound Interest Without a Calculator

Published on Sunday, March 08, 2015 in math, mental math, money, self improvement

Something about the challenging nature of calculating compound interest keeps drawing me back, as in my *Mental Financial Wizard* post and my recent *Estimating Compound Interest* post. Or, maybe I'm just greedy.

In either case, here's yet another way to get a good estimate of interest compounded over time. It's a little tricky to do in your head alone, so you'll probably prefer to work this one out on a sheet of paper.

It turns out that compound interest is based on the binomial theorem. This means we can use relatively simple math concepts from Pascal's Triangle (also based on the binomial theorem). The method I'm about to teach you has its roots in the approach used to work out coefficients in *The Easy Peasy Binomial Expansion Trick* (jump down to the paragraph which reads, *"So now comes the part where the coefficients for each term are written. This is very easy to do with the way we set up our example."*).

What we're going to be estimating is the total percentage of interest alone. Once this is done, you can calculate the original investment into the problem. As a first example, let's work out 5% interest per year for 10 years. To keep things simple, we'll work with 5% as if it represented 5, instead of 0.05.

To get a starting point multiply the interest rate by the time, as if you were working out simple interest. In our 5% for 10 years example, we would simply multiply 5 × 10 = 50. You need to make a table with that number expressed two ways: As a standard number, as as a fraction over 1. For this example, the first row of the table would look like this:

Number | Fraction |
---|---|

50 | ^{50}⁄_{1} |

From here, there are 2 repeating steps, which repeat only as many times as you wish to carry them.

**STEP 1:**You're going to create a new fraction in the next row, based on the existing fraction. Take the existing fraction,

*increase*the denominator (the bottom number of the fraction) by 1, and

*decrease*the numerator by the amount of the annual interest.

In our example, starting from

^{50}⁄

_{1}, we'd increase the denominator by 1, turning it into

^{50}⁄

_{2}, and then decrease the denominator by 5, because we're dealing with 5% interest, to give us

^{45}⁄

_{2}. The table, in this example, would now look like this:

Number | Fraction |
---|---|

50 | ^{50}⁄_{1} |

^{45}⁄_{2} |

**STEP 2:**Take the number from the previous row, and multiply this by the new fraction, in order to get a new number for the current row. Divide the result by 100, and write this number down in the new row. This can seem challenging without a calculator, but if you think of a fraction as simply telling you to divide by the denominator and multiply by the numerator, it becomes simpler.

Continuing with our example, we'll multiply the number from the previous row (50) times our new fraction (

^{45}⁄

_{2}). That's 50 × 45 ÷ 2 = 25 × 45 = 1,125. 1,125 ÷ 100 = 11.25, so we add that number to the new row like this:

Number | Fraction |
---|---|

50 | ^{50}⁄_{1} |

11.25 | ^{45}⁄_{2} |

From here, we can repeat steps 1 and 2 as many times as we like, depending on what kind of accuracy is needed. Repeating step 1 one more time, we get this result (do you understand how we got to

^{40}⁄

_{3}?):

Number | Fraction |
---|---|

50 | ^{50}⁄_{1} |

11.25 | ^{45}⁄_{2} |

^{40}⁄_{3} |

After repeating step 2, we work out 11.25 × 40 ÷ 3 = 11.25 × 4 × 10 ÷ 3 = 45 × 10 ÷ 3 = 450 ÷ 3 = 150. Don't forget, as always, to divide by 100, which gives us 1.5 for the new row:

Number | Fraction |
---|---|

50 | ^{50}⁄_{1} |

11.25 | ^{45}⁄_{2} |

1.5 | ^{40}⁄_{3} |

Most of the time, I stop the calculations when the number in the bottommost row is somewhere between 0 and 10. I find this is enough accuracy for a decent estimate.

Once you've stopped generating numbers, all you need to do to estimate the interest percentage is add up everything in the the

*Number*column! In our above example, we'd add 50 + 11.25 + 1.5 to get 62.75. In other words, 5% for 10 years would yield roughly 62.75% interest. If we run the actual numbers through Wolfram|Alpha, we see that the actual result is about 62.89% interest. That's not bad for a paper estimate!

Back in 2012, a question was posted at math stackexchange which could've benefitted from this technique. In under 3 minutes, answer the following multiple choice question without using a calculator or log tables:

Someone invested $2,000 in a fund with an interest rate of 1% a month for 24 months. Consider it to be compounded interest. What will be the accumulated value of the investment after 24 months?Let's use this technique to work this out:

A) $2,437.53

B) $2,465.86

C) $2,539.47

D) $2,546.68

E) $2,697.40

Number | Fraction |
---|---|

24 | ^{24}⁄_{1} |

2.76 | ^{23}⁄_{2} |

0.2024 | ^{22}⁄_{3} |

Hmmm...24 + 2.76 + 0.2024 = 26.9624, so that would give us about a 26.96% return, or a little less than 27%. Multiplying this by 2,000 is easy, since we can multiply by 2, then 1,000. This lets us know there must be just under $540 in interest on that $2,000. A and B are way too low, E is way too high, and D is just over $540 in interest. That eliminates every answer except C. Sure enough, Wolfram|Alpha confirms that $2,539.47 is the correct answer!