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# Cosines rule: labelling sides

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1. Hi

I am trying to help my son with igcse revision and have just taught myself the cosine rule (this particular example was finding unknown angle when three sides given) but I have a very basic question still bugging me if anyone could help confirm please;
When labelling the sides a,b,c am I right in thinking that the side opposite the angle becomes 'c' because it's opposite cosx and then the a and b can be labelled either way? If not, how do I know which to label a, b and c please?
2. (Original post by Aimfire)
Hi

I am trying to help my son with igcse revision and have just taught myself the cosine rule (this particular example was finding unknown angle when three sides given) but I have a very basic question still bugging me if anyone could help confirm please;
When labelling the sides a,b,c am I right in thinking that the side opposite the angle becomes 'c' because it's opposite cosx and then the a and b can be labelled either way? If not, how do I know which to label a, b and c please?
All sides are a b c correct? so opposite side a you would have angle A same with b and c.

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3. Thank you. I'm getting really muddled. I understand what you've said about the triangle illustrated. This is why i am stuck (bear in mind I did this 26 years ago and had to resit my GCSE to get a C so I'm not the best!). The triangle we are trying to solve is PQR so effectively the P, Q and R of a, b and c. That's okay, I can use these letters but it's not like a b and c stand for any particular side but I've read that you have to order the sides in the formula correctly??

I have actually solved the triangle and got the correct answer but want to be able to tell my son how to label sides in the exam 😊
4. I think I'm just being really dense because maybe it makes no difference which I label a b or c because the formula works out the same as you are just squaring each length and adding then subtracting...
5. (Original post by Aimfire)
Thank you. I'm getting really muddled. I understand what you've said about the triangle illustrated. This is why i am stuck (bear in mind I did this 26 years ago and had to resit my GCSE to get a C so I'm not the best!). The triangle we are trying to solve is PQR so effectively the P, Q and R of a, b and c. That's okay, I can use these letters but it's not like a b and c stand for any particular side but I've read that you have to order the sides in the formula correctly??

I have actually solved the triangle and got the correct answer but want to be able to tell my son how to label sides in the exam 😊
You can assign the letters at random (such as P being the side a, Q being the side b and R being the side c), however the angle opposite to the side of the triangle labelled 'a' must be the angle A, the angle opposite the side 'b' must be the angle B and the angle opposite the side 'c' must be the angle C. As long as each side is given a separate letter (in lower case) and you follow that rule, you should be fine. I've attached an image to use as a reference.

6. (Original post by drinktheoceans)
You can assign the letters at random (such as P being the side a, Q being the side b and R being the side c), however the angle opposite to the side of the triangle labelled 'a' must be the angle A, the angle opposite the side 'b' must be the angle B and the angle opposite the side 'c' must be the angle C. As long as each side is given a separate letter (in lower case) and you follow that rule, you should be fine. I've attached an image to use as a reference.

Thank you. I know it's simple and I'm being ridiculous. I think I can use your example to work this out now. My non maths brain gets thrown by little things like different letters as logic just eludes me!

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