# Help with trigonomic equation...

• Jul 18th 2011, 04:58 PM
lolcheelol
Help with trigonomic equation...
i am supposed to find all solutions in [0,2pi). the original equation is

4sin^2x - 3sinx - 2 = 0

i started off using the quadratic formula and got

3 +/- sqrt|-3^2 - 4*4*-2 / 2*4
after simplifying all these through i was left with 3 +/- sqrt41/8

this is where i am confused and really have no idea where to continue, there really isn't a problem to explain this in the book so any help would be great.
• Jul 18th 2011, 05:06 PM
pickslides
Re: Help with trigonomic equation...

$\displaystyle \sin^{-1}\left( \frac{3+ \sqrt{41}}{8}\right)$

and

$\displaystyle \sin^{-1}\left( \frac{3- \sqrt{41}}{8}\right)$

What do you get?
• Jul 18th 2011, 05:17 PM
lolcheelol
Re: Help with trigonomic equation...
do you mind explaining how to do that?
• Jul 18th 2011, 05:24 PM
pickslides
Re: Help with trigonomic equation...
I accidently edited post #2
• Jul 18th 2011, 05:30 PM
lolcheelol
Re: Help with trigonomic equation...
when i do http://latex.codecogs.com/png.latex?...%7D%5Cright%29 i get domain error

when i do the other one i get -25.1754 rounded
• Jul 18th 2011, 06:06 PM
skeeter
Re: Help with trigonomic equation...
Quote:

Originally Posted by lolcheelol
when i do http://latex.codecogs.com/png.latex?...%7D%5Cright%29 i get domain error

when i do the other one i get -25.1754 rounded

do you understand how the inverse trig functions work and how to interpret what the calculator is telling you?

Pauls Online Notes : Calculus I - Review : Trig Equations with Calculators, Part I
• Jul 18th 2011, 07:23 PM
lolcheelol
Re: Help with trigonomic equation...
Quote:

Originally Posted by skeeter
do you understand how the inverse trig functions work and how to interpret what the calculator is telling you?

Pauls Online Notes : Calculus I - Review : Trig Equations with Calculators, Part I

yes i do, but usually i get stuff like like sin^-1(sqrt3/2) and ill end up with pi/3. so im not exactly sure how this answer is supposed to be written out. i can't find anything like it in my notes and in the book :/
• Jul 18th 2011, 08:02 PM
skeeter
Re: Help with trigonomic equation...
Quote:

Originally Posted by lolcheelol
yes i do, but usually i get stuff like like sin^-1(sqrt3/2) and ill end up with pi/3. so im not exactly sure how this answer is supposed to be written out. i can't find anything like it in my notes and in the book :/

you're used to solving equations whose solutions are unit circle values ... this equation requires the use of a calculator, hence the reason for the link. have a look.
• Jul 18th 2011, 08:23 PM
lolcheelol
Re: Help with trigonomic equation...
awesome, thanks a lot for the help. i had done a little bit of that in my notes, but nothing that explained. GREATLY appreciated!