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[music] You've already seen a few different techniques for how to multiply

quickly. Some of those techniques include

quartersquares, using logarithms or a sliderule.

And that method of prosthaphaeresis, which involves arc cosine.

So yeah, there's a ton of different techniques that I can use to multiply two

numbers quickly. But what if I could already multiply

quickly, but I wanted to divide quickly? What if I wanted to calculate 1 divided by

b? Well, we can use Newton's method to do

this. Now, how we're going to use Newton's

method. Well, Newton's method is really a trick

for finding zeroes of a function. So, if I want to approximate one divided

by b, the reciprocal of b, what I really want to find is some function where that

number is a zero of the function. Newton's method will help me approximate

that zero and consequently help me approximate 1 over b.

There's a ton of different choices that are possible for such a function.

Well, here's one choice. The function f of x equals 1 over x minus

b. And let's just check, yeah, if I evaluate

this function at 1 over b, that's 1 over 1 over b minus b.

But 1 over 1 over b, the reciprocal of the reciprocal of b, is just b.

And b minus b is in fact 0. So, this function f does have 1 over b as

a 0. Now let's apply Newton's method.

So how are we going to do this? Here is the formula for Newtons' method,

right? This is the iterative formula, my next

guess is my old guess minus this fraction. The function evaluate of my old guess

divided by the derivative evaluate of my old guess.

And when this works, it's marching me closer to a 0 of the function.

And here's the good function for doing division.

This function has a 0 at 1 over b. So, if I can get closer and closer to a

zero of this function, I'm actually computing 1 over b, which is what I want

to do. So, let's try to rewrite this formula just

by using this. And the first step here is to

differentiate this. So, let's differentiate this.

Well, the differential goes back to x. So, the derivative of 1 over x is minus 1

over x squared, and the b here is the constant.

So, I have to include it. So, this is the derivative of f.

Now, I'll use that to try to make this Newton's method formula look a little bit

nicer. So, x of n plus 1 is x of n minus the

function evaluated, x of n, which is 1 over x of n minus b, divided by the

derivative evaluator x of n, which is minus 1 over x of n squared.

Now, to kill off this denominator, let's multiple by minus x sub n squared divided

by minus x sub n squared. That's just a sneaky version of 1, but it

manages to kill off the denominator simplify this a bit.

I've got x sub n. I've got minus x sub n squared times 1

over x sub n, that leaves me with a minus x sub n.

And I've got a negative b, negative x sub n squared, that's a plus b x sub n

squared. I'm subtracting this.

So, I've got x sub n plus x sub n, that's two x sub n's, minus b x of n squared.

And I'll factor out an x of n, so x of n times 2 minus bx of n.

And what I've got here now is an iterative formula that, at least when it works.

Applying this iterative formula is going to move me closer and closer to one over

b. But this formula doesn't involve division,

it only involves multiplying and subtracting.

So, this is a way to approximate 1 over b without ever dividing, right?

Just multiplying and subtracting over and over again.

Then, we'll make this even more concrete. Let's set b equals 7 so I can try to

approximate 1 over 7. So here we go.

B will be 7. So, we're trying to approximate 1 7th

using Newton's method. So, I make an initial guess.

And I'll make my initial guess 1 10th and make some of the arithmetic work out, a

little bit more reasonably. And I'm going to start with this initial

guess, and I'll use this formula we just derived to improve this initial guess to a

hopefully better guess. So, I take my original guess, 1 10th.

Multiply it by 2 minus b is 7, times my previous guess of 1 10th.

I can calculate this. This is 1 10th times, instead of 2 I'll

write 20 10th. And instead of 7 times a 10th, I'll write

7 10th. I got 20 minus 7 10th, so that's 13 10th.

So, 1 10th times 13 10th, that's 13 100th and that's a slightly better as to the

value of, of 1 7th, but I can now repeat this process using this formula again.

So, I'll get a better guess here x sub 2 by starting with my previous guess, which

is 13 100th. Multiply that by 2 minus 7 times my

previous guess of 13 100th. I can calculate what this is.

So, 13 100th. Instead of 2, I'll write 200 100th, that's

another name for 2 minus. Instead of 7 times 13, I'll multiply that

out. I'll get 91 100th.

Now, 200 minus 91, that's 109 100th times 13 100th.

And 13 times 109 is 1417, and 100 times 100 is 10,000.

So, that's an even better approximation to the actual value of 1 7th.

And I could appreciate the process again. The arithmetic gets, you know, sort of

awful. But the point here isn't so much, you

know, that I can actually do this on paper, or that I would want to do this on

paper. But that this procedure only involves

adding, subtracting, and multiplying, and yet it ends up calculating a division

problem. Well, how close are we to the actual value

of 1 7th? 1 7th is about 0.142857 and it keeps on

going. So yeah, we're doing really quite well.

I mean, 0.14 is a pretty good guess as to the actual value of 1 7th.

This method has a name. So, this technique usually goes by the

name Newton-Raphson Division. And we just saw that it works for finding

recipricols, but if you can find recipricols and you can multiply, you can

do division in general. Alright, if you wanted to approximate 3

7th using this technique, well, you just point out that 3 7th is 3 times 1 7th,

right? And imagine that you can multiply, add,

subtract, but the division is hard. Okay, well a moment ago, we approximated 1

7th, alright? We approximated 1 7th to be about 1,417

over 10,000. This was the second stage in the

approximation. And 3 times 1,417 is 4251 over 10,000.

Which isn't so far off of 3 7th, right? The point though is that, you know, if

division, but if division that only involves repeated adding, subtracting, and

multiplying.