Question 1189398: The answer to the inequality log(x2-7x) < log(3-x) + log(2 is
(A) -1 < x < 0
(B) x > 7 or x < 0
(C) x > 7)
or -1 < x < 6
(D) -1 < x < 6
(E) -1 < x < 3
Answer by ikleyn(52787) (Show Source):
You can put this solution on YOUR website! .
The answer to the inequality log(x2-7x) < log(3-x) + log(2) is
(A) -1 < x < 0
(B) x > 7 or x < 0
(C) x > 7)
or -1 < x < 6
(D) -1 < x < 6
(E) -1 < x < 3
~~~~~~~~~~~~~~~
First, the domain for this inequality is the set of real numbers x such that
x^2 -7x > 0 and 3-x > 0,
or { x < 0 & x < 3 } U {x > 7 & x < 3 }.
Of the last two sets under the union sign, the second set {x > 7 & x < 3 } is empty;
so, the domain is the set { x < 0 }. (1)
Next, the given inequality is equivalent to
log(x^2 -7x) < log (2(3-x))
which implies (due to the monotonicy of the logarithm function)
x^2 - 7x < 6 - 2x.
What follows, is the solution procedure for this inequality.
x^2 - 5x - 6 < 0,
(x-6)*(x+1) < 0.
The last inequality has the solution set { -1 < x < 6 }. (2)
To get the final answer, we shoud take the intersection of the set (2) with the domain set (1).
The intersection is the set { -1 < x < 0 }, or, in the interval form, (-1,0).
ANSWER. The solution to given inequality is the set { -1 < x < 0 }, or, in the interval form, (-1,0).
Solved, answered, and explained.
I checked my solution visually, using plotting calculator www.desmos.com.
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Never write the optional answers all in one line: place each optional answer in separate line.
I was forced to fix / (to re-edit) the entire your post.
Now it makes sense, which it did not make before.
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