# Triangle edge length from 2 edge lengths and angle

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Ok this is pretty embarassing... This problem looks like its as easy as eating cake. However I can't seem to solve it which has me very frustrated. Looking at the diagram, I know length L1 and L2 and i know the angle Theta; What i want to find out is the length x. I tried splitting it into two right angle triangles, and setting up a set of simulataneous equations. All was going well, but then i ended up with same really complicated equation that was not easy to solve. I must be missing something simple, what is the easiest way to do this?

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Well, since the picture is broken, I can't realy be sure what your goal is but guessing that x is the side opposite to theta this will be how it looks:

x*x = L1*L1 + L2*L2 - 2*L1*L2*cos(theta)

Which gives you the side x to be what?

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Hum yes picture seems broken now strange, website must be down...

Anyway, thanks yes cosine rule... exactly what I needed. Thank you for your help. High school maths, my brain must be decaying...

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