Trong tam giác cân, do a = b nsup> + b2 = 2a2 = 2ab}}:

c 2 = 2 a 2 ( 1 − cos ⁡ γ ) . {\displaystyle c^{2}=2a^{2}(1-\cos \gamma ).\;}

hay

cos ⁡ γ = 1 − c 2 2 a 2 {\displaystyle \cos \gamma =1-{\frac {c^{2}}{2a^{2}}}}