For each of the following functions, use the test given in Appendix 2 to determine whether it

Question:

For each of the following functions, use the test given in Appendix 2 to determine whether it is convex, concave, or neither.
(a) f (x) = x1x2 – x21 – x22
(b) f (x) = 3x1 + 2x21 + 4x2 + x22 – 2x1x2
(c) f (x) = x21 + 3x1x2 + 2x22
(d) f (x) = 20x1 + 10x2
(e) f (x) = x1x2