Let $R'$ be a tensor of order 4 in a riemannian manifold $M$ defined by: $R'(W,Z,X,Y)=\langle W,X \rangle \langle Z,Y\rangle - \langle Z,X\rangle \langle W,Y\rangle $
And let $R$ be the curvature tensor of $M$, if we have $R=KR'$, how to conclude that $\nabla_U R=(UK)R'$?
By definition: $\nabla_U R(W,Z,X,Y)=\nabla R(W,Z,X,Y,U)=U(R(W,Z,X,Y))-R(\nabla_UW,Z,X,Y)- \ldots$ and stuck here.