The Student Room Group

Cosine Rule - what am I doing wrong?

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:

cos⁡θ=a2+b2−c22ab\cos \theta = \frac{a^2 + b^2 - c^2}{2ab}

Yet I got:

cos⁡−1θ=26.43∘\cos^{-1} \theta = 26.43^{\circ}

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?
(edited 13 years ago)
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:

cos⁡θ=a2+b2−c22ab\cos \theta = \frac{a^2 + b^2 - c^2}{2ab}

Yet I got:

cos⁡−1θ=26.43∘\cos^{-1} \theta = 26.43^{\circ}

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.
Reply 2
Avatar for Dededex
Dededex
OP
10
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? :smile:
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? :smile:


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.
mathsisfun to the rescue.

http://www.mathsisfun.com/algebra/trig-cosine-law.html
Reply 5
Avatar for Dededex
Dededex
OP
10
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.

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you