GCSE Maths help (lengths of a triangle) Watch

iPad User
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#1
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#1
In the triangle ABC
AC= 5CM
BC= 8CM
Angle ACB= 75 degrees

Calculate the lengh of AB
Give your answer correct to 3 significant figures.

So I'm stumped. Its not trigonometry because there's no right angle, and its not Pythagoras?
Thank you
(Do you need a picture?)
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stayd001
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#2
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Use the cosine rule
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Davott
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#3
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SinA/A = SinB/B

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#4
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To find the original value of x^2 you square it, how do you find the original value of x^3?
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Stormwhite
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(Original post by iPad User)
To find the original value of x^2 you square it, how do you find the original value of x^3?
I'm perplexed, what do you mean? x=\sqrt[3] {x^3} if that's what you mean.

As to the original problem, it's cosine rule. a^2 = b^2+c^2-2bc\cos \hat {A}

In this particular method, a=AB, b=BC, c=AC
and \hat {A}=75

Sub those in to the equation.
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Lukedavidhopkins1
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It's definitely cosine rule, as you have two lengths and the angle between them
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#7
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(Original post by Stormwhite)
I'm perplexed, what do you mean? x=\sqrt[3] {x^3} if that's what you mean.

As to the original problem, it's cosine rule. a^2 = b^2+c^2-2bc\cos \hat {A}

In this particular method, a=AB, b=BC, c=AC
and \hat {A}=75

Sub those in to the equation.
But I don't know the value of C, I tried I think I got 3 something... I rearranged the formula.
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Stormwhite
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(Original post by iPad User)
But I don't know the value of C, I tried I think I got 3 something... I rearranged the formula.
What? No, no, where I've written BC and AC, they are the lengths. You are trying to find a, you have b=8, c=5, \hat A =75

Put those values into the cosine rule equation.
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ArsLongaVitaBrevis
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#9
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Law of cosines.
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MathematicsKiller
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Cosine rule: http://www.math-mate.com/chapter60_2.shtml

That is how to use it.
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Sokka
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cosine rule

Spoiler:
Show
8.26cm
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Sokka
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(Original post by iPad User)
To find the original value of x^2 you square it, how do you find the original value of x^3?
i think you're slightly confused.
Ok here, take an example.

5^2 = 25
original value is the square ROOT of 25 which is 5 right?

5^3 = 125
original value of 125 is the cube root this time. Cube root is basically finding the number that multiplies 3 times to get 125 - which is obviously 5, because 5 x 5 x 5 = 125
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