# Taylor Series and error

zcd
I'm doing some review over summer before starting college, and one of the practice exams has a question pertaining to the remainder of a taylor series

## Homework Statement

Show that $$\left|\cos{(1+x)}-\{\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})\}\right|<\frac{1}{15000}$$ for $$|x|<0.2$$

## Homework Equations

The two equations I somewhat remember are
$$R_n(x)=\frac{f^{(n+1)}(c)}{(n+1)!}(x-a)^{(n+1)}$$
and
$$|R_n(x)|\leq{}M_n\frac{r^{n+1}}{(n+1)!}$$
where M (iirc) is the largest value of the nth derivative in the area of convergence?

## The Attempt at a Solution

Here's where I'm stuck; my class in high school never fully went over error of taylor approximation because it wasn't part of the curriculum, so we were given the equations without any actual application. To make matters worse, I last took AP calculus BC over a year ago, so this is still very rusty. Can anyone point me in the right direction?

Homework Helper
Gold Member
I'm doing some review over summer before starting college, and one of the practice exams has a question pertaining to the remainder of a taylor series

## Homework Statement

Show that $$\left|\cos{(1+x)}-\{\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})\}\right|<\frac{1}{15000}$$ for $$|x|<0.2$$

## Homework Equations

The two equations I somewhat remember are
$$R_n(x)=\frac{f^{(n+1)}(c)}{(n+1)!}(x-a)^{(n+1)}$$
and
$$|R_n(x)|\leq{}M_n\frac{r^{n+1}}{(n+1)!}$$
where M (iirc) is the largest value of the nth derivative in the area of convergence?

## The Attempt at a Solution

Here's where I'm stuck; my class in high school never fully went over error of taylor approximation because it wasn't part of the curriculum, so we were given the equations without any actual application. To make matters worse, I last took AP calculus BC over a year ago, so this is still very rusty. Can anyone point me in the right direction?

If you expand cos(1+x) in a Taylor series about x = 0 up through the term containing x3 you will have:

cos(1+x) = (terms from constant through x3) + R3

where R3 is the error term if you stop with x3. So you will have

|cos(1+x) - (terms from constant through x3)| = |R3|, where

$$R_3(x)=\frac{f^{(4)}(c)}{(4)!}(x-0)^{(4)}$$

If you write out the series you will see it is exactly what is given. All you have to do is estimate the remainder. It tells you that |x| ≤ .2 and you will find it easy to overestimate f(4)(x). Just be brave and work it out. Don't use any decimals, stick with fractions.

zcd
Still getting my feet wet again so correct me if I'm wrong :P

$$\left|\cos{(1+x)}-\{\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})\}\right|=\left|\cos{(1)}((1-\frac{x^2}{2}+R_3(x))-(1-\frac{x^2}{2}))-\sin{(1)}((x-\frac{x^3}{3!}+R_3(x))-(x-\frac{x^3}{3!}))\right|$$
$$=\left|\cos{(1)}R_3(x)-\sin{(1)}R_3(x)\right|$$

since $$|R_n(x)|\leq{}M_n\frac{r^{n+1}}{(n+1)!}$$ and Mn=1 for sin and cos, $$|R_3(x)|\leq{}\frac{(\frac{1}{5})^4}{4!}=\frac{1}{15000}$$

plugging it all back in,
$$\left|\cos{(1)}\frac{1}{15000}-\sin{(1)}\frac{1}{15000}\right|<\frac{1}{15000}$$

is that right?

Homework Helper
Gold Member
Still getting my feet wet again so correct me if I'm wrong :P

$$\left|\cos{(1+x)}-\{\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})\}\right|=\left|\cos{(1)}((1-\frac{x^2}{2}+R_3(x))-(1-\frac{x^2}{2}))-\sin{(1)}((x-\frac{x^3}{3!}+R_3(x))-(x-\frac{x^3}{3!}))\right|$$
$$=\left|\cos{(1)}R_3(x)-\sin{(1)}R_3(x)\right|$$

since $$|R_n(x)|\leq{}M_n\frac{r^{n+1}}{(n+1)!}$$ and Mn=1 for sin and cos, $$|R_3(x)|\leq{}\frac{(\frac{1}{5})^4}{4!}=\frac{1}{15000}$$

plugging it all back in,
$$\left|\cos{(1)}\frac{1}{15000}-\sin{(1)}\frac{1}{15000}\right|<\frac{1}{15000}$$

is that right?

The only thing right is your estimate for R3. The rest is just jumbled confusion.

Just write the Taylor series for cos(1+x) using the terms up through x3 like I suggested. You know, calculate f(0), f'(0), f''(0) etc. and write the series. This doesn't involve the remainder. Rearrange it to get the expression you are given. Write it as suggested in my previous post.

zcd
I think I get it

$$\cos{(1+x)}=\cos{(1)}-x\sin{(1)}-\frac{x^2}{2}\cos{(1)}+\frac{x^3}{3!}\sin{(1)}+...$$
$$\cos{(1+x)}=\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})+R_3(x)$$
$$\cos{(1+x)}-\left[\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})\right]=R_3(x)$$
$$\left|\cos{(1+x)}-\left[\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})\right]\right|=\left|R_3(x)\right|$$
$$\left|\cos{(1+x)}-\left[\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})\right]\right|\leq\frac{1}{15000}$$

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Homework Helper
Gold Member
I think I get it

$$\cos{(1+x)}=\cos{(1)}-x\sin{(1)}-\frac{x^2}{2}\cos{(1)}+\frac{x^3}{3!}\sin{(1)}+...$$
$$\cos{(1+x)}=\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})+R_3(x)$$
$$\cos{(1+x)}-\left[\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})\right]=R_3(x)$$
$$\left|\cos{(1+x)}-\left[\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})\right]\right|=\left|R_3(x)\right|$$
$$\left|\cos{(1+x)}-\left[\cos{(1)}(1-\frac{x^2}{2})-\sin{(1)}(x-\frac{x^3}{3!})\right]\right|\leq\frac{1}{15000}$$

how does the =< turn into <?

Good job. Look at the limits on x. Are you given strict inequality in your estimate?

zcd
Ahh I see. Thanks for all the help :)