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# C2 - Trigonometric Equations help. watch

1. Hi,

Can someone please help me with this C2 question? I have looked at the answer but am still unsure. Mark scheme is also attached!

Thanks!
Attached Images

2. Working?
3. Hi,

As cos(θ + 75) = 0, I first took the inverse cos of cos(θ + 75) so:

θ + 75 = cos-1(0)

Therefore θ + 75 = 90 and therefore θ = 15. However this is not the answer. Could you please explain how to do this question

Thanks
4. What you do for question a) is I'm gonna use Z as theta

cos(z+75) = 0
z + 75 = Cos^-1(0)
z + 75 = 90
z = 90 - 75
z = 15

If you then look on the cos graph you can see where the next one will be, or you can just know that with cos your first number will be your Point Value i.e. 15, then you do 360 - P.V so your next answer for Z will be 345, an that is where that comes from.

For Part B)
you do the same, again with Z as Theta, however, this time you want to change your boundaries, so because it's 2z you will times the
0 < z < 360 by 2 so, 0 x 2 = 0, 360 x 2 = 720 so your new boundaries are... 0 < z < 720
so Sin2z = 0.7
2 z = Sin^-1(0.7)
2z = 44.4, 135.6, 404.4, 495.6
Then divide these by two and that gives you the answers they've said
5. the answer scheme looks wrong...

your version is right... θ + 75 = 90 etc....
6. (Original post by Oedg)
What you do for question a) is I'm gonna use Z as theta

cos(z+75) = 0
z + 75 = Cos^-1(0)
z + 75 = 90
z = 90 - 75
z = 15

If you then look on the cos graph you can see where the next one will be, or you can just know that with cos your first number will be your Point Value i.e. 15, then you do 360 - P.V so your next answer for Z will be 345, an that is where that comes from.
Hi,

7. (Original post by DhylanP - CRGS)
Hi,

That one I'm a bit unsure about, let me just try and work it out, I may have got the cos part slightly wrong. Im currently doing C3 and C4 (A2) so I've forgotten some of the stuff just need to refresh
8. (Original post by DhylanP - CRGS)
Hi,

And by the way the MS is wrong, its +15 not -15
9. (Original post by DhylanP - CRGS)
Hi,

I think, once again, the MS is wrong, however, I'm not 100% sure. If you can see from the image, you can see it starts at 15 and the next one it hits is 345 so I done know why they put 225.

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Updated: February 13, 2018
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