# Homework Help: Simple Sinx Expansion

1. Jan 10, 2013

### bmb2009

1. The problem statement, all variables and given/known data

Expand sinx about the point x= pi/4. Hint: Represent the function as sinx= sin(y+pi/4) and assume y to be small

2. Relevant equations

3. The attempt at a solution

I thought the problem was simply asking to expand sinx with the McLauran expansions about the point pi/4 and get something like... (x-pi/4) - ((x-pi/4)^3)/3! + ((x-pi/4)^5)/5!.....so on and so forth.

But the hint throws me off? What does that mean? Any help?

Thanks

2. Jan 10, 2013

### haruspex

The hint is telling you to do precisely what you thought. It's just trying to make life simpler by substituting y for (x-pi/4) everywhere.

3. Jan 10, 2013

### jbunniii

Your expansion clearly cannot be correct: if you plug in x = pi/4, you get 0. But sin(pi/4) is not 0.

What happens if you apply a trig identity to sin(y + pi/4)?

4. Jan 10, 2013

### lurflurf

$$\sin(x+h) \sim \sum^\infty_{k=0} \frac{h^k}{k!} \sin^{(k)}(x)=\sum^\infty_{k=0} \frac{h^k}{k!} \sin(x+k \, \pi/2)=\sum^\infty_{k=0} \frac{h^k}{k!} (\sin(x)\cos(k \, \pi/2)+\cos(x)\sin(k \, \pi/2))$$

$$\sin^{(k)}(x)$$
means the kth derivative of sine evaluated at x which we know to be
$$\sin(x+k \, \pi/2)$$

Last edited: Jan 11, 2013
5. Jan 11, 2013

### jbunniii

I'm not sure what good that does. This isn't a Taylor series in x. I guess it's a Taylor series in h, but centered at h=0, which isn't what is asked for.

I think the hint is intended to lead to the following:
$$\sin(x) = \sin(y + \pi/4) = \sin(y) \cos(\pi/4) + \cos(y) \sin(\pi/4)$$
And we presumably know the Taylor series for $\sin(y)$ and $\cos(y)$.

6. Jan 11, 2013

### lurflurf

^It does not matter what variables are used.
In general h is not zero that is an uninteresting case.

These all mean exactly the same thing.
$$\sin(x+h) \sim \sum^\infty_{k=0} \frac{h^k}{k!} \sin^{(k)}(x)=\sum^\infty_{k=0} \frac{h^k}{k!} \sin(x+k \, \pi/2)=\sum^\infty_{k=0} \frac{h^k}{k!} (\sin(x)\cos(k \, \pi/2)+\cos(x)\sin(k \, \pi/2)) \\ \sin(a+b) \sim \sum^\infty_{n=0} \frac{b^n}{n!} \sin^{(n)}(a)=\sum^\infty_{n=0} \frac{b^n}{n!} \sin(a+n \, \pi/2)=\sum^\infty_{n=0} \frac{b^n}{n!} (\sin(a)\cos(n \, \pi/2)+\cos(a)\sin(n \, \pi/2)) \\ \sin(\mathrm{rock}+\mathrm{paper}) \sim \sum^\infty_{\mathrm{scissors}=0} \frac{\mathrm{paper}^\mathrm{scissors}}{\mathrm{scissors}!} \sin^{(\mathrm{scissors})}(\mathrm{rock})= \sum^\infty_{\mathrm{scissors}=0} \frac{\mathrm{paper}^\mathrm{scissors}}{\mathrm{scissors}!} \sin(\mathrm{rock}+\mathrm{scissors} \, \pi/2)=\sum^\infty_{\mathrm{scissors}=0} \frac{\mathrm{paper}^\mathrm{scissors}}{\mathrm{scissors}!} (\sin(\mathrm{rock})\cos(\mathrm{scissors} \, \pi/2)+\cos(\mathrm{rock})\sin(\mathrm{scissors} \, \pi/2))$$

In the given exercise we can take
x=rock+paper
y=paper
pi/4=rock
k=scissors

to give
$$\sin(x)=\sin(\pi/4+y) \sim \sum^\infty_{k=0} \frac{y^k}{k!} \sin^{(k)}(\pi/4)=\sum^\infty_{k=0} \frac{y^k}{y!} \sin(\pi/4+k \, \pi/2)=\sum^\infty_{k=0} \frac{y^k}{k!} (\sin(\pi/4)\cos(k \, \pi/2)+\cos(\pi/4)\sin(k \, \pi/2))$$