Why Does the Divergence of aRab + bRgab Vanish at b = -1/2a?

[solveforX]

I'm reading gravitation and having trouble with one of the exercises.

aR_ab+bRg_ab is the general tensor

the exercise asks to show that the divergence of this tensor vanishes if and only if b=-1/2a
how do I go about solving this problem?

 

[jfy4]

I'm reading gravitation and having trouble with one of the exercises.

aR_ab+bRg_ab is the general tensor

the exercise asks to show that the divergence of this tensor vanishes if and only if b=-1/2a
how do I go about solving this problem?

Check this out: http://mathworld.wolfram.com/ContractedBianchiIdentities.html"