math question?

Question

Consider the function f(x)=2x+9x^(-1). For this function there are four important intervals:(-inf, A], [A, B), (B, C] and [C, INF) where A, and C are the critical numbers and the function is not defined at B. Find A, B, C
For each of the following intervals, tell whether f(x) is increasing (type in INC) or decreasing (type in DEC). Find
(-INF, A], [A, B), (B, C] and [C, INF)
Note that this function has no inflection points, but we can still consider its concavity. For each of the following intervals, tell whether f(x) is concave up (type in CU) or concave down (type in CD). Find (-INF, B), and (B, INF)

Answer 1

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http://answers.yahoo.com/question/index;_ylt=Aiu_Y2F4zYx4ITnt.lD6buzty6IX?qid=20070623220522AAHCSTI

Answer 2

f(x) = 2x + 9/x
f'(x) = 2 - 9/x^2 = 0 for critical points
B = 0
2x^2 = 9
x = ± (3/2)√2
A = - (3/2)√2
B = 0
C = (3/2)√2
f'(x) = 2 - 9/x^2
(-INF, A], INC
[A, B), DEC
(B, C] DEC
[C, INF) INC
f''(x) = 18/x^3
(-INF, B), CD
(B, INF), CU