A formula known as law of cosine (sometimes also referred to as Carnot's rule) states that, in any triangle ABC, you have
[tex] \overline{AB}^2 = \overline{BC}^2+\overline{AC}^2-2\overline{AC}\overline{BC}\cos(\alpha)[/tex]
where [tex]\alpha[/tex] is the angle between AC and BC. This formula generalizes Pythagoras' theorem, since it doesn't require the triangle to be right. In your case, we have
[tex] [tex]AB = \sqrt{\dfrac{1}{4}+\dfrac{1}{9}-2\cdot\dfrac{1}{2}\cdot\dfrac{1}{3}\cdot\cos(100)} = \sqrt{\dfrac{13}{36}-\dfrac{\cos(100)}{3}} \approx 0.65[/tex]
