15. In a triangle $A B C$, Let $\angle C = n / 2$. If $r$ is the inradius and $R$ is the circum-radius of the triangle, then $2 ( r + R )$ is equal to : (A) $a + b$ (B) $b + c$ (C) $c + a$ (D) $a + b + c$
15. In a triangle $A B C$, Let $\angle C = n / 2$. If $r$ is the inradius and $R$ is the circum-radius of the triangle, then $2 ( r + R )$ is equal to :\\
(A) $a + b$\\
(B) $b + c$\\
(C) $c + a$\\
(D) $a + b + c$\\