# How would one calculate the cosine of an obtuse angle?

How would you calculate the cosine of an obtuse triangle's largest angle?

-
"How would you calculate" is not clear enough. You did post a diagram, which is helpful. But you did not indicate what is known about the triangle. Is it the three sides? If that is the case, say so, and give the measurements. Is it one angle and two sides? The more specific you are, the more quick and relevant will be the response. –  André Nicolas Jun 15 '11 at 18:42

Cosine=adjacent/hypotenuse is only true in right triangles, as that is the only time there is a hypotenuse. $\cos \theta$ is well defined for $\theta \gt 90^{\circ}$ and we have $\cos \theta=-\cos (180^{\circ}-\theta)$.