Which of following sets is not a convex set?

[karan]

Which of following sets is not a convex set?
(a) X = {(x1,x2) : 3*x1^2 + 2*x2^2 <= 6}
(b) X = {(x1,x2) : x1>=2, x2<=3}
(c) X = {(x1,x2) : x1*x2<=1,x1>=0,x2>=0}
(d) X = {(x1,x2) : x1^2 + x2^2<=4}

 

[royhaas]

Re: Convex set

Hint: It's easy if you graph the regions.