# Find period of function

1. Aug 20, 2008

### fr33pl4gu3

f(x) = - sin(x/5 - pi)

the period for this is -pi correct, since, k taken out it is 1, and by divide -pi into 1, the answer will be -pi only, correct??

2. Aug 20, 2008

### mgb_phys

Re: Period

I don't think you can have a negative period!
To find the period pick a value of x (say zero) and then work out what is the next value of x that will give the same value of f().
Does adding the same constant to both values of X do anything?

3. Aug 20, 2008

### fr33pl4gu3

Re: Period

Does it gives the same pi??

4. Aug 20, 2008

### fr33pl4gu3

Re: Period

5. Aug 20, 2008

### mgb_phys

Re: Period

The period of sin() is pi ie sin(0)=sin(pi)

But in terms of X, since you divide X by 2 what value of X do you need for the function to give the same value as for x=0?

6. Aug 20, 2008

### fr33pl4gu3

Re: Period

What is the exact period of f(x) (in radians)? (Recall p radians is equivalent to 180°; p is obtained by entering: Pi or pi.)

What does the question actually wants anyway?? how to calculate period in radians.

7. Aug 20, 2008

### fr33pl4gu3

Re: Period

zero??

8. Aug 20, 2008

### Avodyne

Re: Period

This is not correct. A function $f(x)$ is periodic with period $p$ if
$$f(x+p)=f(x)$$
for all real $x$. For $\sin(x)$, this is true for $p=2\pi$, and not true for $p=\pi$, because $\sin(x+\pi)=-\sin(x)$, not $\sin(x)$.

9. Aug 20, 2008

### fr33pl4gu3

Re: Period

yes, and the answer is 10pi, by getting the 2pi divide 1/5 to get the answer. I get confuse that i need to multiply by 5 so to get b=5, but when i try 1/5, it's done.

10. Aug 20, 2008

### Avodyne

Re: Period

Correct!

11. Aug 20, 2008

### Staff: Mentor

Re: Period

in this function, f(x) = - sin(x/5 - pi), the -pi would be a phase shift.

In general for sin (wt + q), w = 2pi f = 2 pi/T, where T is the period, and q is the phase shift.

12. Aug 21, 2008

### HallsofIvy

Staff Emeritus
Re: Period

No, the period of sin(x) is 2pi.

fr33pl4gu3. your function is -sin(x/5- pi). When x/5- pi= 0, what is x? When x/5- pi= 2pi, what is x? The difference between those is the period.