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1. Guys can someone help with the following question..
From a point A, a boat sails due north for 7km to B. The boat leave point B and moves at a bearing of 100 degrees for 10km until it reaches C. Calculate the distance of C from A.

I keep getting the wrong answer. This is my working out:

SinC/7 = Sin100/10
C= 43.6

Therefore angle B is 180-(100+43.6)
= 36.4

AC/sin36.4 = 10/sin100

AC = 10sin36.4 / sin100
= 6.026 cm

Where have I gone wrong??

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2. (Original post by Taraah)
SinC/7 = Sin100/10
C= 43.6
This first statement is not true.

Draw a diagram.. You cannot do this question using the sine rule - you have no pairing angle and lengths.
3. The book tells me to use the cosine rule, but I still get the answer wrong. I've written the question exactly the way its written in the book. I drew a diagram but now I'm really confused

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4. (Original post by Taraah)
The book tells me to use the cosine rule, but I still get the answer wrong. I've written the question exactly the way its written in the book. I drew a diagram but now I'm really confused

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You should be able to work out the angle ABC by noting that angles on a straight line add up to give 180 degrees.

Then use the fact that

5. (Original post by Taraah)
The book tells me to use the cosine rule, but I still get the answer wrong. I've written the question exactly the way its written in the book. I drew a diagram but now I'm really confused

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If you've drawn the sketch correctly you should have a value for a, b and an angle which (if the ship goes on a bearing of 100 from and the angle of a straight line is 180 what would be the angle inside the triangle?) http://gyazo.com/336ae70eb7cd08fc049fc564ef9c676b
6. note that bearings are measure from north, clockwise
7. $c^{2}=a^{2}+b^{2}-2ab\cos\theta$
Rearrange:
$b^{2}=a^{2}+c^{2}-2ac\cos\theta$
Things to bear in mind (no pun intended):
- Bearing are measured north clockwise meaning angle B has a measurement of 80 degrees.
- Angles are opposite to sides.
- Plug your numbers in using brackets and round.
8. (Original post by Secret.)
If you've drawn the sketch correctly you should have a value for a, b and an angle which (if the ship goes on a bearing of 100 from and the angle of a straight line is 180 what would be the angle inside the triangle?) http://gyazo.com/336ae70eb7cd08fc049fc564ef9c676b
Hey thank you!! I drew the diagram incorrectly, thanks for helping i got the right answer now, finally!

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9. (Original post by Secret.)
If you've drawn the sketch correctly you should have a value for a, b and an angle which (if the ship goes on a bearing of 100 from and the angle of a straight line is 180 what would be the angle inside the triangle?) http://gyazo.com/336ae70eb7cd08fc049fc564ef9c676b
Hey is it possible that you can help me with a another question:
A helicopter flies on a bearing of 200 degrees from A to B, where AB= 70km. It then flies on a bearing of 150 degrees from B to C, where C is due south of A. Work out the distance of C from A.

I properly know how to work it out, but I struggle with drawing the sketches.. Can you please tell me how to sketch would look like? Because I know I'm not drawing it right.

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10. (Original post by Taraah)
Hey is it possible that you can help me with a another question:
A helicopter flies on a bearing of 200 degrees from A to B, where AB= 70km. It then flies on a bearing of 150 degrees from B to C, where C is due south of A. Work out the distance of C from A.

I properly know how to work it out, but I struggle with drawing the sketches.. Can you please tell me how to sketch would look like? Because I know I'm not drawing it right.

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Hi, sorry for the late reply, I'm guessing it would look something like: http://gyazo.com/78d9fa8f6386c96686a99cbe70af496d

It also shows that C is due south of A
hope this helps.
(I kept the north lines so you can see how I got the drawing, the angles are not exact and don't have to be just approximate e.g. for 200 I used the north line as the angle of 180 and added approx 020 to it.

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Updated: March 23, 2013
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