# Finding a Trig Equation

• Jun 17th 2008, 01:42 PM
FORK
Finding a Trig Equation
Here is a picture of my question, just click on it to enlarge it. I need the equation to be sin. Here is an example from another question of the format of the formula.

5sin30(x-1)+7

And here is the question

http://i25.tinypic.com/9rp2fo.jpg

Thanks alot.
• Jun 17th 2008, 02:07 PM
galactus
You can run it through Excel or a lot of calculators and they'll do sine regressions. I used my ol' TI-92 and got:

$\displaystyle y=1.085605sin(.907418x+.209669)-0.229008$

For the second part of the problem, set the equation equal to 0.5 and solve for x.
• Jun 17th 2008, 02:11 PM
FORK
K, i tried it and it didnt give the proper output numbers. I need the formula to include the axis, amplitude etc, if that helps.
• Jun 17th 2008, 02:17 PM
galactus
That's all in your equation.The amplitude is obviously 1.085605.

The equation I gave gives the proper output. Not right on the money, but very close. That is as close as you can get.

For instance, if x=1.25, we get .829

Compare to .83 That's good.
• Jun 17th 2008, 06:12 PM
FORK
It dosent work... For .5 i get a -21. The amplitude should equal .425... and there is no phase shift so i dont know what youre doing.
• Jun 17th 2008, 06:50 PM
FORK
• Jun 18th 2008, 12:43 PM
galactus
Are you sure you're not in degrees. When I use .5, I get .439

.45 is what is in the chart. That is very close. Looking again, I do not see anything wrong. The data matches very close.

Using my equation we get

0 is -.003

.5 is .44

.75 is .61

1.25 is .83

2.25 is .61

3 is -.003

I do not know how you got -21 for x=.5

Those look close to me.

I suppose I am misunderstanding.

If you have something like $\displaystyle y=a\cdot{sin(bx+c)}$, then the period is $\displaystyle \frac{2\pi}{|b|}$ and the phase shift is $\displaystyle \frac{-c}{b}$. The amplitude is |a|