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# Cosine Rule - what am I doing wrong? Watch

1. Question:

4) In a triangle XYZ find angle Z when YZ= 4.7cm, XZ = 10.5 cm and XY = 8.9 cm.

So I know it's cosine so I used:

Yet I got:

Which is wrong; somehow I think it's because I've labelled the sides wrong or something but I didn't think that mattered - could I have some help please?
2. (Original post by Dededex)
Question:

4) In a triangle XYZ find angle Z when YZ= 4.7cm, XZ = 10.5 cm and XY = 8.9 cm.

So I know it's cosine so I used:

Yet I got:

Which is wrong; somehow I think it's because I've labelled the sides wrong or something but I didn't think that mattered - could I have some help please?
Draw a triangle and label the vertices X Y Z. Now label the sides. Call the angle at Z theta and call the side opposite this vertex c. It doesn't matter which of the remaning sides you call a and which one you call b.
3. (Original post by Mr M)
Draw a triangle and label the vertices X Y Z. Now label the sides. Call the angle at Z theta and call the side opposite this vertex c. It doesn't matter which of the remaning sides you call a and which one you call b.
Oh right - how come the one opposite the angle has to be c? Is that just the general rule? Or does that only apply to this specific question?
4. (Original post by Dededex)
Oh right - how come the one opposite the angle has to be c? Is that just the general rule? Or does that only apply to this specific question?
If you use the Cosine Rule in the form you have given it, c is always facing the angle you are finding and a and b are the sides that meet to form the angle.
5. mathsisfun to the rescue.

http://www.mathsisfun.com/algebra/trig-cosine-law.html
6. (Original post by Mr M)
If you use the Cosine Rule in the form you have given it, c is always facing the angle you are finding and a and b are the sides that meet to form the angle.
Oh right thanks alot Mr M.

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