Mickey Mouse?

If $a,b,$and $c$are the side lengths of $\triangle ABC$opposite to angles $A,B,$and $C,$respectively, and $r_{1},r_{2},$and $r_{3}$相应的exradii,那么找到价值吗of

$\frac {b-c}{r_{1}} + \frac {c-a}{r_{2}} + \frac{a-b}{r_{3}}.$