# Thread: triangle give 2sides and a angle -cosine?

1. ## triangle give 2sides and a angle -cosine?

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give triangle ABC with a=7 b=x c=5 and angle A = 60 degrees. Find x

2. Apply the cosine rule:

$a^2=b^2+c^2-2bc\cos A$

Then, $49=x^2+25-5x\Rightarrow x^2-5x-24=0$

Find the roots and choose the positive one.