hello In this MTW Ex 21.16, MTW refers to box 14.5 in which the Riemann curvature scalar contains a second derivative of the scale factor a for the same Friedmann metric. Now using the equation suggested by MTW, i.e. eq 21.77, the value of tr(K)2 - tr(K2) would be zero since K is multiple of the unit tensor. This is said, what remains of Eq 21.77 is simply : -G00 = 1/2R00 but this would not yield the equation to be proved since we still have a second derivative of a. I'm confused. Note: In Landau's section 107 however, the value of the G00 found will give the equation to be proved despite the difference of the values of the curvature tensors.