If x, y and z are positive real numbers, such that x+y+z=a, then

(a) 1/x + 1/y + 1/z > 9/a (b) (a-x) (a-y) (a-z) > 8xyz (c) (a-x) (a-y) (a-z) < 8/27 a³

Please give full explanation .