#Trigonometry- cosine

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pure sleet
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I'm really confused about the mark scheme for this question. Why did they remove cos when using the cosine rule? They got 4cm but I've got 1cm

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humble pike
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@pure sleet you added an extra unnecessary cos

the value √5/3 was calculated by doing sohcahtoa on that right angled triangle to find
cos(θ) = a/h = √5/3

so when using the cosine rule, you replace cos(θ) with √5/3

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the angle is roughly 41.8⁰ not √5/3, cosine of the angle is √5/3