If a, b, c are rational numbers but not zero, a+b+c=0 , what is the

If a, b, c are rational numbers but not zero, a+b+c=0 , what is the value of

\dfrac{1}{b^2+c^2-a^2} +\dfrac{1}{c^2+a^2-b^2} +\dfrac{1}{a^2+b^2-c^2}