Solution to exercise

The y-isoquant is given by
y = z11/2 + z21/2,
or
z2 = (y z11/2)2
so that
MRTS(z1, z2) = (y z11/2)z11/2 = (z2/z1)1/2.
(Remember that the MRTS is the absolute value of the slope of an isoquant.)