Grade 12IIT JEE Entrance Exam
For what value of x is x^2-3x-18 positive?
For what value of x is x^2-3x-18 positive?
1 Answer
Vikas TU
Approved Tutor Answer11 Years agoYou have gotten this far:
+++++(-3)----------(6)++++++
Consider the intervals (-∞, -3),(-3,6),(6, ∞)
Choose 'any' one point from each of the intervals.
(-∞, -3) : choose x=-4
x^2-3x-18 where x=-4 = 10 > 0 (positive)
(-3,6): choose x=0
x^2-3x-18 where x=0 = -18
(6, ∞) : choose x=7
x^2-3x-18 where x=7 = 10 > 0 (positive)
The intervals where x is positive are:
(-∞, -3) and (6, ∞)
(-∞, -3) U (6, ∞) is the answer
or
x 6
+++++(-3)----------(6)++++++
Consider the intervals (-∞, -3),(-3,6),(6, ∞)
Choose 'any' one point from each of the intervals.
(-∞, -3) : choose x=-4
x^2-3x-18 where x=-4 = 10 > 0 (positive)
(-3,6): choose x=0
x^2-3x-18 where x=0 = -18
(6, ∞) : choose x=7
x^2-3x-18 where x=7 = 10 > 0 (positive)
The intervals where x is positive are:
(-∞, -3) and (6, ∞)
(-∞, -3) U (6, ∞) is the answer
or
x 6
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