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Can anyone help me with C2 maths on raidans?

Find in radians correct to 2 decimal places, the two smallest positive values of θ for which sin(2θ +1/3π) = 0.123. I've worked out 2.68 but I figure out how to get to the other answer.
If you could help that would be great.:smile:
Hi, these are the steps I used:
(I'll have to use 'a' for theta as I cannot get that symbol)

sin(2a+1/3pi)=0.123
2a+1/3pi=sin^-1(0.123)
2a+1/3pi=0.123
2a=-0.925
a=-0.462

You then use the symmetry of the sin(a) graph:
(-pi)+0.462= -2.68

I hope two things: that's correct and that it helps you if it is. I cannot see how you got +2.68 (unless there is a typo in the original message or my working out)
Reply 2
Original post by Jasminea
Find in radians correct to 2 decimal places, the two smallest positive values of θ for which sin(2θ +1/3π) = 0.123. I've worked out 2.68 but I figure out how to get to the other answer.
If you could help that would be great.:smile:


If you call x=2θ+π3x = 2\theta + \frac{\pi}{3} and then solve sinx=0.123\sin x = 0.123 then we have x=sin10.123+2πx=\sin^{-1} 0.123 + 2\pi and then x=πsin1(0.123)+2πx = \pi - \sin^{-1} (0.123) +2\pi.

Back-sub: 2θ+π3=sin10.123+2π2\theta+ \frac{\pi}{3} = \sin^{-1} 0.123 + 2\pi (solve for theta) for the first solution and 2θ+π3=πsin1(0.123)+2π2\theta + \frac{\pi}{3} = \pi - \sin^{-1} (0.123) +2\pi for the second solution.
Reply 3
Original post by ErraticPhysicist
Hi, these are the steps I used:
(I'll have to use 'a' for theta as I cannot get that symbol)

sin(2a+1/3pi)=0.123
2a+1/3pi=sin^-1(0.123)
2a+1/3pi=0.123
2a=-0.925
a=-0.462

You then use the symmetry of the sin(a) graph:
(-pi)+0.462= -2.68

I hope two things: that's correct and that it helps you if it is. I cannot see how you got +2.68 (unless there is a typo in the original message or my working out)


It asks for positive values.
Original post by Zacken
It asks for positive values.


Ah yes, I see. Your answer looks correct, well done :smile:
Reply 5
Original post by Zacken
If you call x=2θ+π3x = 2\theta + \frac{\pi}{3} and then solve sinx=0.123\sin x = 0.123 then we have x=sin10.123+2πx=\sin^{-1} 0.123 + 2\pi and then x=πsin1(0.123)+2πx = \pi - \sin^{-1} (0.123) +2\pi.

Back-sub: 2θ+π3=sin10.123+2π2\theta+ \frac{\pi}{3} = \sin^{-1} 0.123 + 2\pi (solve for theta) for the first solution and 2θ+π3=πsin1(0.123)+2π2\theta + \frac{\pi}{3} = \pi - \sin^{-1} (0.123) +2\pi for the second solution.


Ok I can solve it but how did you get 2Pi? Other than that, thanks for help.
Reply 6
Original post by Jasminea
Ok I can solve it but how did you get 2Pi? Other than that, thanks for help.


Well, trigonometric functions are two-pi periodic, i.e: they repeat themselves after every two pi radians.

So if x is a solution to a trig equation, then so is x + 2pi, so is x + 2pi + 2pi, so is x + 2pi + 2pi + 2pi, etc... because sin (x) = sin (x + 2pi) since the function just repeats itself after 2pi, so if I add 2pi, it changes nothing.
Reply 7
Ok. Yeah that makes sense thanks.
Reply 8
Original post by Jasminea
Ok. Yeah that makes sense thanks.


No problem.
Reply 9
Original post by Zacken
No problem.

Hi, did u ever get 0.99 as a value, bcoz that's the other answer in the back of my textbook? I can't get that answer. It may be wrong.
Reply 10
Original post by Jasminea
Hi, did u ever get 0.99 as a value, bcoz that's the other answer in the back of my textbook? I can't get that answer. It may be wrong.


It's nor a mistake.

2θ+π3=π2sin1(0.123)\displaystyle 2\theta + \frac{\pi}{3} = \frac{\pi}{2} - \sin^{-1} (0.123).

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