A bar made of material whose Young's modulus is equal to E and Poisson's ratio to

Question:

A bar made of material whose Young's modulus is equal to E and Poisson's ratio to µ is subjected to the hydrostatic pressure p. Find:
(a) The fractional decrement of its volume;
(b) The relationship between the compressibility β and the elastic constants E and µ. Show that Poisson's ratio µ cannot exceed 1/2.