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# How to Calculate the Length of this? watch

1. How do you calculate the length of AD? Do you use Trigonometry? If so I thought Trigonometry could only be used for Right-Angled Triangles. thank you in advance!
2. You can do this with the cosine rule. If you know what that is, try and use it. If not, look it up or ask again!
3. What level is this at? It looks like it could just be a really long winded trig question near the end of a paper?
4. (Original post by Pangol)
You can do this with the cosine rule. If you know what that is, try and use it. If not, look it up or ask again!
Thanks! I haven't learnt about the cosine rule yet so thanks for pointing it out!
5. (Original post by Tubbz)
What level is this at? It looks like it could just be a really long winded trig question near the end of a paper?
It's from an Old O-Level Past Paper. It was quite an early question but i found out i need to use the cosine rule to solve it, but thanks for the help!
6. (Original post by zattyzatzat)
It's from an Old O-Level Past Paper. It was quite an early question but i found out i need to use the cosine rule to solve it, but thanks for the help!
Fair enough, is the cosine rule is in the syllabus then it makes sense to use that. Otherwise you could probably solve it by brute forcing it with basic trig.
7. The cosine rule is certainly on the current GCSE syllabus, so I'm sure it would have been on an old O Level one!
8. (Original post by Pangol)
The cosine rule is certainly on the current GCSE syllabus, so I'm sure it would have been on an old O Level one!
I did my GCSEs and A Levels a while ago, pretty hard to remember what fell which side of the line.
9. (Original post by zattyzatzat)

How do you calculate the length of AD? Do you use Trigonometry? If so I thought Trigonometry could only be used for Right-Angled Triangles. thank you in advance!
Yes,Zattyzatzat,the usual rules for trigonometry use right angles.To use trigonometry on triangles that don't have right angles you have to split them into two right triangles like this:

.

Other rules that are useful to apply are the sin and cos rules.Look up the proofs of these,they involve lots of circles!

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