Proving concave functions and convex sets

May 2013
4
0
Hi,
having some problems with these questions..

1)Prove that the function y=f(X1,X2) ; y= X1^1/2 + X2^1/2 is a strictly concave function on R++

2)
follow up question to 1.
D = {X1,X2 | X1^1/2 + X2^1/2 = y}
Prove that D is a convex set.