Problem

In a triangle with one angle

\frac{2\pi}{3}

, the lengths of the sides form an A.P. If the length of the greatest side is 7 cm, the radius of the circumcircle of the triangle is a.

\frac{7\sqrt{3}}{3}

cm b.

\frac{5\sqrt{3}}{3}

cm c.

\frac{2\sqrt{3}}{3}

cm d.

\sqrt{3}