- [Instructor] Estimating sine of 0.4
using a Maclaurin polynominal, what is the least degree
of the polynominal that assures an error smaller than 0.001?
So what are we talking about here?
Well, we could take some function
and estimate it with an nth degree Maclaurin polynomial,
in fact, we could talk more generally
about a Taylor polynominal, but let's just say
this is an nth degree Maclaurin polynomial,
but this isn't going to be a perfect approximation,
there's going to be some error or some remainder.
And so we could call this the remainder
of that nth degree Maclaurin polynomial,
and it's going to be dependent for any given x.
Now if we want to use the specifics of this
exact problem, we could phrase it this way.
We want to say, look, if we're taking the sine of 0.4
this is going to be equal to our Maclaurin,
our nth degree Maclaurin polynomial evaluated at 0.4
plus whatever the remainder is
for that nth degree Maclaurin polynomial evaluated at 0.4,
and what we really want to do is figure out for what n,
what is the least degree of the polynomial?
So what is,
let me do this in a different color.
So we want to figure out what is the smallest n,
what is the smallest n
such that the remainder
of our nth degree Maclaurin polynomial evaluated at 0.4
is less than this number, is less than 0.001?
So this is just another way of rephrasing the problem.
And the way that we can do it is we can use
something called the Lagrange error round
and we have other videos that prove it,
this is often also called Taylor's Remainder Theorem.
And I'll first write it out and I'll try
to explain it while I write it out, but it'll actually
become a lot more concrete when we work it out.
So Taylor's Remainder Theorem tells us,
or Legrange error bound tells us that
if the n plus oneth derivative of our function,
so f, so this is our n plus oneth derivative
of our function, if the absolute value of that
is less than or equal to some M
for an open interval,
open interval, containing
where our polynomial is centered,
in this case it's zero, we're gonna use the Maclaurin case,
so it's containing zero, and the x,
or zero and x, the x that we care about
in this particular video is 0.4,
but I'll say generally for any x,
so if this is true, if our n plus oneth derivative
of our function, if the absolute value of it
is less than or equal to M, over an open interval
containing where we're centered,
this would be C if we're talking about the general case,
and x, so this x right over here, then,
and this is the Legrange, this is the part that's useful,
we can say that the remainder is bounded,
the remainder for that nth degree polynomial.
So this is the n plus oneth derivative,
that's bounded, then we can say the remainder
for the nth degree polynomial that approximates our function
is going to be less than or equal to that M
times x to the n plus one,
over n plus one factorial.
So how do we apply that to this particular problem?
Well think about the derivatives of sine,
we know that the absolute value of sine
is less than or equal to one, its derivative is cosine of x,
the absolute value of that is going to be bounded,
is going to be less than or equal to one,
so no matter how many times we take the derivative
of sine of x, the absolute value of that derivative
is going to be less than or equal to one.
So we could write generally that for this particular f of x,
so let's say, for this particular f of x right over here,
we could say that the absolute value
of the n plus oneth derivative, evaluated any x,
is going to be less than or equal to one.
And this is the case for where f is sine,
where f is sine of x, and this is actually
going to be true over any interval,
it doesn't even have to be over some type
of a restricted interval where we can do this.
So we know that this is our M,
that sine and its derivatives are all bounded,
or their absolute values are bounded by one.
And so them we have our M and we can apply
the Legrange error bound, so we can say
that the remainder of our nth degree Maclaurin approximation
at 0.4, so our x in this particular case is 0.4,
we don't have to do it generally for any x here,
is going to be less than or equal to,
our M is one, so I won't even write that down,
our x is 0.4, 0.4 to the n plus one,
to the n plus one, over n plus one factorial.
And we're taking the absolute value of this whole thing.
This is Legrange error bound, and we want to figure out,
if we can figure out a situation where this
is less than 0.001, then this for sure
is going to be less than 0.001, because the remainder
is less than this, or less than or equal
to this, which is less than that.
So how do we do that, how do we figure out
the smallest n where this is going to be true?
Well, we can just try out some ns and keep increasing
until this thing actually becomes smaller than that thing.
So let's do it, all right, I'm gonna set up a table here.
So let me do it relatively cleanly.
So let's do, this is going to be our n,
and then this is going to be,
this is going to be 0.4 to the n plus one,
over n plus one factorial,
so let's try it when n is equal to one.
Well then this is going to be 0.4 to the two,
so it's 0.4 squared over two factorial,
this is 0.16 over two,
which is equal to 0.08, that's definitely
not less than one thousandth here.
So let's try n equals two, when n equals two,
it's gonna be 0.4 to the third power,
over three factorial, and that's equal to,
what is that, zero point,
I'm gonna need three digits behind the decimal,
0.064 over six, well, this is a little bit more
than 0.01, so our n isn't large enough yet.
So let's try three, so this is going to be 0.04
to the three plus one, so that's going to be
to the fourth power, over four factorial,
and let's see, that is going to be equal to,
this is going to be, let's see, we're gonna have
four digits behind the decimal, so 0.0256
over 24, this is, we're almost there,
this is a little bit, this is going to be a little bit more
than 0.001, so that doesn't do the trick for us.
So I'm guessing already that n equals four
is gonna do the trick, but let's verify that.
So this is going to be 0.04, or 0.4, I should say,
to the fifth power, over five factorial,
and what is this equal to?
Let's see, four to the fifth is 1024,
I'm gonna have five numbers behind the decimal
and I'm gonna divide it by five factorial, which is 120.
And let's see, this right over here, yes,
this for sure is less than 0.001,
this is definitely less than a thousandth right over here.
So we see that when n is equal to four,
so we can say that the remainder for our fourth degree
polynomial, fourth degree Maclaurin polynomial, evaluated
at x equals 0.4 is for sure going to be less than 0.001.
So there you go, that is the least degree of the polynomial
that assures an error smaller than one thousandth.