seeking your help with figure

Note : 

Area of triangle  AOB = (1/2)c * r

Area of triangle AOC = (1/2)b * r

Area of triangle BOC  = (1/2)a * r

So....areae of triangle ABC  = " K"   =

(1/2)c*r  + (1/2)b*r  + (1/2)a *r  =

(1/2)r [ a + b + c ]  =

r  [(a + b + c)  / 2 ]  =

r * s