Casting Out Nines - Numberphile
Dr James Grime and a handy trick for checking calculations. Ditch the nines... More links & stuff in full description below ↓↓↓ Some extra footage from this interview: https://youtu.be/wQsQ...
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Today I'm gonna show everyone a trick that can check your arithmetic. But I think this is something that everyone should know. I think every school kid should know this. I think every adult should know this, and I don't think everyone does know it. It is called casting out nines. It's a method to check your arithmetic– it's what they used in the old days before calculators. And let's say we're doing something like 2039 plus 172 plus, whatever, 1218 plus 3091. Let's say we're doing something like that. Let's add these up. I'm just gonna add it up in the traditional way, I'm not doing anything clever here. So you add up column by column, okay, I reckon it's 6520. But what if I've done it wrong? ...
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Today I'm gonna show everyone a trick that can check your arithmetic. But I think this is something that everyone should know. I think every school kid should know this. I think every adult should know this, and I don't think everyone does know it. It is called casting out nines. It's a method to check your arithmetic– it's what they used in the old days before calculators. And let's say we're doing something like 2039 plus 172 plus, whatever, 1218 plus 3091. Let's say we're doing something like that. Let's add these up. I'm just gonna add it up in the traditional way, I'm not doing anything clever here. So you add up column by column, okay, I reckon it's 6520. But what if I've done it wrong? ...
