# Problem about set theory

Set A and B are defined as follows:

A={x: 1<=x<9 } and B={y: y<=5}.

a) Are Sets A and B convex? Are they closed? Are the bounded?

b) Is the set A∩B convex, closed and/or bounded?

Could you please explain more?

This one:

unbounded( y can approach to -∞)

### 1 Answer

- CRebeccaLv 610 years agoFavorite Answer
(a)

A is convex {x1, x2 in [1, 9), a+b=1, a, b>=0, then ax1+bx2 in [1, 9) }

not closed(not contain the limit point x=9),

bounded ( lower bound=1 , upper bound=9)

B is convex, closed(contain any limit points), unbounded

(b)

A∩B is convex (intersection of convex sets A and B)

not closed [not contain the limit point (9, 5) ]

unbounded( y can approach to -∞)

2010-10-09 13:38:54 補充：

Which one?

2010-10-10 19:44:39 補充：

A∩B={(x,y)| 1 <= x < 0, y <= 5}

If A∩B is bounded then there exists M>0 such that x^2+y^2 < M for all (x,y) in A∩B.

i.e. A∩B is enclosed by a circle, it is impossible, since y can approach to -∞.

Thus, A∩B is unbounded.

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