
Kindly forward me the link to the 2012 DSE paper detailed solutions, if available.
I have a doubt in Question 45 of the paper..
f(x, y) = x + y + xy where x, y E R++. For c E R++, let
us define,
L = {f(x, y) E R++ f(x, y) <= c}
U = {f(x, y) E R++ f(x, y) >= c}
I = {f(x, y) E R++ f(x, y) = c}
Which of the above sets are convex?
(a) L
(b) U
(c) I
(d) All of them
