# maths question - cosine ruleWatch

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#1
ill post the question asap
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#2
0
4 weeks ago
#3
(Original post by sqrt of 5)
ill post the question asap

You need to find x^2. For your equation in terms of x for AB. So I suggest making your equation Cos PBQ = (100 100 - y^2) / 200 which simplifys to 1 - y^2/ 200 as you have done.

Then you need to find y. You can do this by finding the length of AC. Form another triangle ABC and use the fact they are congruent to identify the angles of them. Then use the sine rule to find y.

Have a go at doing that now, and if you’re still stuck I’ll try to help a bit more.
Last edited by STEM.lover; 4 weeks ago
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#4
here's what I did
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4 weeks ago
#5
You need to use cosine rule twice.
I remember this question, last question on my GCSE paper :P
(Original post by sqrt of 5)
here's what I did
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#6
(Original post by IslamJoynul)
You need to use cosine rule twice.
I remember this question, last question on my GCSE paper :P
i did XD
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4 weeks ago
#7
Yup that’s right.

Another way of doing it that I used:

You need to make a triangle ABC, where angle B is 30 degrees. Angle c and a are both 75 degrees. (180- 30)/2. Then using the sine rule you can get length AC where AC = PQ. Then you use the Cos rule as seen above.
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4 weeks ago
#8
(Original post by sqrt of 5)
This damn question.... It came up when I did gcses, I still can't do it as an A-level student lol.I had no clue how to answer it lol + the markscheme does not make any sense
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#9
(Original post by STEM.lover)
Yup that’s right.

Another way of doing it that I used:

You need to make a triangle ABC, where angle B is 30 degrees. Angle c and a are both 75 degrees. (180- 30)/2. Then using the sine rule you can get length AC where AC = PQ. Then you use the Cos rule as seen above.
ill defo try this method because it seems easier
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