字幕表 動画を再生する 英語字幕をプリント - This video is about the ridiculous way we used to calculate Pi. For 2000 years the most successful method was painstakingly slow and tedious, but then Isaac Newton came along and changed the game. You could say he speed-ran Pi and I'm gonna show you how he did it. But first Pi with pizzas. Cut the crust off of pizza and lay it across identical pizzas. And you'll find that it goes across three and a bit pizzas, this is Pi. The circumference of a circle is roughly 3.14 times its diameter but Pi is also related to a circles area, area's just Pi R squared. But why is it Pi R squared? Well cut a pizza into really thin slices and then form these slices into a rectangle. Now the area of this rectangle is just length times width. The length of the rectangle is half the circumference because there's half the crust on one side and half on the other, so the length is Pi R. And then the width is just the length of a piece of pizza which is the radius of the original circle. So area is Pi R times R, area is Pi R squared. So the area of a unit circle then is just Pi, keep that in mind because it'll come in handy later. So what was the ridiculous way we used to calculate Pi? Well, it's the most obvious way. It's easy to show that Pi must be between three and four, take a circle and draw a hexagon inside it, with sides of length one. A regular hexagon can be divided into six equal lateral triangles. So the diameter of the circle is two. Now the perimeter of the hexagon is six and the circumference of the circle must be larger than this, so Pi must be greater than six over two. So Pi is greater than three. Now draw a square around the circle, the perimeter of the square is eight which is bigger than the circles circumference, so Pi must be less than eight over two. So Pi is less than four. This was actually known for a thousands of years. And then in 250 BC, Archimedes improved on the method. - So first he starts with the hexagon, just like you did and then he bisects the hexagon to dodecagon. So that's a 12 sided, regular 12 sided shape. And he calculates its perimeter, the ratio of that perimeter to the diameter will be less than Pi. He does the same thing for a circumscribed 12-gon and finds an upper bound for Pi. The calculations now become a lot more tricky because he has to extract square roots and square roots of square roots and turn all these into fractions, but he works out the 12-gon, then the 24-gon, 48-gon and by the time he gets to the 96-gon he sort of had enough, but he gets, in the end he gets Pi to between 3.1408 and 3.1429. So for over 2000 years ago, that's not too bad. - Yeah, that seems like all the precision you'd need in Pi. - Right, so this goes way beyond precision for any practical purpose. This is now a matter of flexing your muscles. This is showing off just how much mathematical power you have, that you can work out a constant like Pi to very high precision. So for the next 2000 years, this is how everyone carried on bisecting polygons to dizzying heights as Pi passed through Chinese, Indian, Persian and Arab mathematicians, each contributed to these bounds along our committee's line. And in the late 16th century, Frenchman Francois Viete doubled a dozen more times than Archimedes, computing the perimeter of a polygon with 393,216 sides only to be out done at the turn of the 17th century by the Dutch Ludolph van Ceulen. He spent 25 years on the effort computing to high accuracy the perimeter of a polygon with two to the 62 sides. That is four quintillion, 611 quadrillion, 686 trillion, 18 billion, 427 million, 387,904 sides. What was the reward for all of that hard work? Just 35, correct decimal, places of Pi. He had these digits inscribed on his tombstone, 20 years later, his record was surpassed by Christoph Grienberger who got 38, correct decimal places. - But he was the last to do it like this - Pretty much. Yeah, because shortly thereafter we get Sir Isaac Newton on the scene. And once Newton introduces his method nobody is bisecting n-gons ever again. The year was 1666 and Newton was just 23 years old. He was quarantining at home due to an outbreak of bubonic plague. Newton was playing around with simple expressions like one plus X, all squared. You can multiply it out and get one plus two X plus X squared. Or what about one plus X all cubed? Well, again, you can multiply out all the terms and get one plus three X plus three X squared plus X cubed. And you could do the same for one plus X to the four or one plus X to the five and so on. But Newton knew there was a pattern that allowed him to skip all the tedious arithmetic and go straight to the answer. If you look at the numbers in these equations the coefficients on X and X squared and so on, well, they're actually just the numbers in Pascal's triangle. The power that one plus X raised to corresponds to the row of the triangle And Pascal's triangle is really easy to make, it's something that's been known from ancient Greeks in Indians and Chinese Persians, a lot of different cultures discovered this. All you do is whenever you have a row you just add the two neighbors and that gives you the value of the row below it. So that's a really quick easy thing you can compute the coefficients for one plus X to the 10 in a second instead of sitting there doing all the algebra. - The thing that fascinated me when I started looking at those old documents was how even like, I don't speak those languages, I don't know those numbers systems and yet it is obvious, it is clear as day that they're all writing down the same thing which today in the Western world, we call Pascal's triangle. - That's the beauty of Mathematics. It transcends culture, it transcends time, it transcends humanity. It's gonna be around well after we're gone and ancient civilizations, alien civilizations we'll know Pascal's triangle. Over time, people worked out a general formula for the numbers in Pascal's triangle. So you can calculate the numbers in any row without having to calculate all the rows before it, for any expression one plus X to the N it is equal to one plus N times X plus N times N minus one X squared on two factorial plus N times N minus one times and N minus two times X cubed on three factorial and so on. And that's the binomial theorem. So binomial, because there's only two terms, one in X by is two, there's two normals and a theorem is that this is a theorem that you can rigorously prove that this formula is exactly what you'll see as the coefficients in Pascal's triangle. - [Alex] So all of this was known in Newton's day already. - Yeah, exactly, everybody knew this. Everybody saw this formula and yet nobody thought to do with it the thing that Newton did with it which is to break the formula. The standard binomial theorem insist that you apply it only when N is a positive integer, which makes sense. This whole thing is about working out one plus X times itself a certain number of times, but Newton says, screw that just apply the theorem. Math is about finding patterns and then extending them and trying to find out where they break. So he tries one plus X to the negative one. So that's one over one plus X. What happens if I just blindly plug in N equals negative one for the right-hand side of the formula? And what you get is the terms alternate back and forth. Plus one minus one, plus one minus one, and so on forever. So that's one minus X, the next term will be a plus X squared, the next one will be a minus X cubed plus X to the fourth minus X to the fifth. So that just alternating series with plus and minus signs as the coefficient. - [Derek] So it becomes an infinite series. - Yeah, that's right. If you, don't a positive integer the binomial theorem, Newton's binomial theorem will give you an infinite sum. - But how do you understand that? Like for all positive integers it was just a finite set of terms and now we've got an infinite set of terms. - Yeah, so what happens is if you have a positive integer you remember that formula, the coefficient looks like N times N minus one times N minus two and so on, when you get to N minus N, if N is a positive integer, you will eventually get there and N minus 10 is zero. So that coefficient and all the coefficients after it are all zero and that's why it's just a finite sum, it's a finite triangle. But once you get outside of the triangle with positive integers, you never hit N minus N because N is not a positive integer, so you get this infinite series. - [Derek] So I think the big question is, does this actually work? Does Newton's infinite series actually give you the value of one over one plus X? - Right, and it might be nonsense. There's lots of math formulas that could break completely when you do this. There's, we have rules for a reason but we should always know the extent to which the rules have a chance of working farther. If you take that whole series and you multiply it by one plus X and you multiply all that out you'll see all the terms cancel, except that leading one. And so that big series times one plus X is one. In other words, that big series is one over one plus X, that's how Newton justified to himself that it makes sense to apply the formula where it shouldn't be applicable. So Newton is convinced the binomial theorem works even for negative values of N, which means there's more to Pascal's triangle above the zeroeth you could add a zero and a one that add to make that first one. And then that row would continue minus one, plus one, minus one, plus one, all the way out to infinity. And outside the standard triangle the implied value everywhere is zero. And this fits with that. The alternating plus and minus ones add to make zero everywhere in the row beneath them. And you can extend the pattern for all negative integers either using the binomial theorem or just looking at what numbers would add together to make the numbers underneath. And here's something amazing. If you ignore the negative signs for a minute these are the exact same numbers arranged in the same pattern as in the main triangle. The whole thing has just been rotated on its side.