Posted by Rachale on Wednesday, July 22, 2009 at 11:34pm.
Solve x(x7)(x9)<0 and express the solution set in interval notation.
A. (–infinity, 0)
B. (–infinity, 0) U (7, 9)
C. (–infinity, 0) U (9, infinity)
D. (–infinity, 9)

Algebra  MathMate, Thursday, July 23, 2009 at 12:14am
It is a cubic, with three real zeroes (0,+7 and +9), therefore the graph crosses the xaxis three times.
The coefficicent of the x³ term is positive, so it increases to the right of the largest root (+9) and decreases to the left of the smallest root (0).
This tells us that (∞,0) is a subset of the solution.
To find the remaining part of the solution, we note that the function is positive between 0 and the next root, and dips below zero again between the last two roots, namely +7 and +9.
Thus the missing interval is (7,9).
Can you take it from here? 
Algebra  Rachale, Thursday, July 23, 2009 at 1:00am
Based upon your explanation, I am goin to say the answer is
B(–infinity, 0) U (7, 9)
? Is this correct? 
Algebra  MathMate, Thursday, July 23, 2009 at 7:16am
Correct!