5.
已知边长分别为\(a\),\(b\),\(c\)的三角形\(ABC\)面积为\(S\),内切圆\(O\)的半径为\(r\),连接\(OA\),\(OB\),\(OC\),则三角形\(OAB\),\(OBC\),\(OAC\)的面积分别为\( \dfrac {1}{2}cr, \dfrac {1}{2}ar, \dfrac {1}{2}br\),由\(S= \dfrac {1}{2}cr+ \dfrac {1}{2}ar+ \dfrac {1}{2}br\)得\(r= \dfrac {2S}{a+b+c}\),类比得四面体的体积为\(V\),四个面的面积分别为\(S_{1}\),\(S_{2}\),\(S_{3}\),\(S_{4}\),则内切球的半径\(R=\) ______ .