Dictionary:Cosine law

In any plane triangle with sides a,b, and c and the angle ${\displaystyle \phi \,}$ between sides a and b,
${\displaystyle c^{2}=a^{2}+b^{2}-2ab\cos \phi .}$
If ${\displaystyle \phi \,}$ > 90°, the cosine is negative and the last term becomes additive. See Figure S-10.