M.Johnson2111
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Hi,

Please can I have some help with this question?

I originally thought the triangle looked like an equilateral so was going to use that idea to find x but not sure if thats correct?

I was then going to use the sine rule but we know two sides and an angle but I cant spot a relationship with them.

Then for the cosine rule we dont know the angle between the sides but we have three sides?
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M.Johnson2111
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mqb2766
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Use the cos rule and formulate and solve a quadratic in x.
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M.Johnson2111
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Thank you
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M.Johnson2111
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Name:  image.jpg
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Size:  160.6 KBNot sure where I’ve gone wrong since 4 does not equal 28!

I’ve redone the question below the box to check for errors but cannot find any?
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M.Johnson2111
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Nevermind, cos 60 does not equal 1!
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M.Johnson2111
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I end up with x = 8.5 and x = -10.5

I know to diregard x = -10.5

But these answers really dont seem correct?
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Muttley79
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(Original post by M.Johnson2111)
I end up with x = 8.5 and x = -10.5

I know to diregard x = -10.5

But these answers really dont seem correct?
Can you post your new working - I don't get those answers.
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M.Johnson2111
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Size:  77.9 KBSorry about photo quality
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M.Johnson2111
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Basically I used the cosine rule

Let a/b = x/x+2

And cos C = cos 60 = (square root 3) / 2 using exact values

Then made this all equal (2squareroot7)^2

And attempted to solve for x there and solved the quadratic I calculated using my graphical
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Pangol
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(Original post by M.Johnson2111)
Basically I used the cosine rule

Let a/b = x/x+2

And cos C = cos 60 = (square root 3) / 2 using exact values

Then made this all equal (2squareroot7)^2

And attempted to solve for x there and solved the quadratic I calculated using my graphical
cos(60) is not root(3)/2. I know that you are expected to know the exact values for things like this, and it is a great help if you can be confident with them, but if in doubt, check!
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M.Johnson2111
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Oh no youre right, I was using the hand trick, its the figures above not below!

cos 60 = 1/2
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mqb2766
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(Original post by M.Johnson2111)
Oh no youre right, I was using the hand trick, its the figures above not below!

cos 60 = 1/2
cos(60) = 1, sqrt(3)/2, 1/2, .... you're going to be running out of fingers soon :-)
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davros
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(Original post by M.Johnson2111)
Oh no youre right, I was using the hand trick, its the figures above not below!

cos 60 = 1/2
I don't even want to know what this is - it sounds disgusting

Personally I only ever remember that sin 30 = 1/2. Once I've got that imprinted on my brain I can very quickly think that "sin 60 must be 'the other one'; cos 30 must be 'the other one'; cos 60 must be the same as sin 30" etc
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Pangol
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This is a nice easy way to remember them. I was shown it by a visiting European student - it's apparently quite well known on the continent, but it was new to me.

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