Find the extreme points in the following polyhedra set:

(a) P ={(x1,x2,x3)|x1 +x2 +x3 1,x1,x2,x3 0}.

(b) P = {(x1, x2, x3, x4)|x1 + x2 + 1 x3 1, x1, x2, x3, x4 0}.