Transcript of Faster than a Calculator by Arthur Benjamin at TEDxOxford
Arthur Benjamin – Mathemagician
Well good afternoon, ladies and gentlemen. My name is Art Benjamin, and I am a mathemagician. What that means is, I combine my loves of math, or I should say maths and magic to do what I call “mathemagics.”
But before I get started, I’ve got a quick question for the audience. By any chance, did anyone – anyone here in the audience happen to bring with them this afternoon a calculator? If you have one, perhaps on your phone or somewhere and you are pretty comfortable using it, raise your hand. I will need a couple of people to help me out here. I see one, two and perhaps one more. Three, the three of you bring out your calculators and join me up here on stage and let’s give these volunteers a nice round of applause. Come on up!
Great. Over on this side please. Awesome. Now, since I have not had the chance to work with these calculators, I need to make sure that they are all working properly. Would somebody get us started by giving us a two-digit number, please? How about a two-digit number?
Oh, that’s fine. 74. And another – how about another two-digit number, how about on this side?
Multiply 74 times 39 on the calculator, make sure you get 2886, or the calculators are not working. Do each of you get 2886? Give them a round of applause.
Now I notice it took some of us a little bit of time to get the answer. That’s okay. I’ll give you a shortcut for multiplying even faster on the calculator. There is something called the square of a number, which most of you know is taking a number and multiplying it by itself. For instance, five squared would be? 25. Six squared would be 36. 73 squared would be something else, yeah.
Now on most of these calculators they have little shortcut buttons that allow you to square numbers even faster. What I’m going to try and do now is to square, and you might test and make sure you could square 5 or 6 with it but what I am going to try and do is to square in my head, three two-digit numbers faster than they can do on their calculators, even using the shortcut method. What I’ll ask is three people, how about in the third row here, one, two, three, each yell out a two digit number and if you would square the first one, the second and third one, I will try and race you to the answer. So quickly, a two-digit number please.
- Great, next. 98, and one more, 26. Would you call out your answers, please?
[Audience: 576, 9604, 676.]
Give them a round of applause.
Let me try to take this one step further. I’m going to try to square some three-digit numbers this time. I won’t even write these down — I’ll just call them out as they’re called out to me. Anyone at all, call out a three-digit number. Anyone on our panel, verify the answer. Now if I get the answer right, give me a big thumbs up, if I make a mistake, let me know and I will try and fix it. A three-digit number, anyone?
576 is 331776.
Yes? Good. How about another three-digit number, sir? Three digit number?
103 is 10609, too easy. Another three-digit number, please?
125 is 15625 but that’s 5 to 6 power, so that was easy too. How about another three digit number sir?
985 is 970225, yes, thumbs up, if it’s right. One more three digit number sir?
Oh, 926 is 857476.
Thank you very much.
Let me try to take this one step further. I’m going to try to square a four-digit number this time. I am not going to beat you to the answer on this one, but I will try to get the answer right. To make this a little bit more random, how about we use the fourth row four people, each of you call out a single digit between zero and nine, that will be the four-digit number that I’ll square.
1, 5, 7, 7
1577, this will take me a little bit of time, so bear with me. 3,486929.. no, don’t tell me. Number was 1577. Oh, wait, wait, 2,486929. [Was everything else right? Thank you very much. What’s one million off, that’s all I ask.
Now, I would attempt to square a five-digit number — and I can — but unfortunately, most calculators cannot.