✔ 最佳答案
1) 5x + 2 <= 17
Isolate x. Subtract 2 from both sides of the inequality, to get
5x <= 15
Divide 5 both sides,
x <= 3
In interval notation, this is written as
(-infinity, 3]
2) For the second inequality, it would be the WRONG step to multiply both sides by (5x + 6), so PLEASE nobody under me do it.
Let me show you instead how it is solved.
1/(5x + 6) > -3
First, move the -3 to the left hand side.
1/(5x + 6) + 3 > 0
Next, merge as a single fraction.
1/(5x + 6) + 3(5x + 6)/(5x + 6) > 0
(1 + 3(5x + 6)) / (5x + 6) > 0
Simplify the numerator.
(1 + 15x + 18) / (5x + 6) > 0
(15x + 19) / (5x + 6) > 0
Obtain our critical values by noting what makes the left hand side equal to 0 or what makes the left hand side undefined.
It is 0 when the numerator is 0, i.e. when
15x + 19 = 0, or x = -19/15
It is undefined when the denominator is undefined, i.e. when
5x + 6 = 0, or x = -6/5 = -18/15
Make a number line consisting of these critical values.
. . . . . . . . . . (-19/15) . . . . . . . . . .(-18/15) . . . . . . . . . . . . . . .
This number line is your sign diagram. You want to test a single value in each region, plug into (15x + 19) / (5x + 6), and determine if the result is positive or negative.
For x < -19/15, test x = -100. Then
(15x + 19) / (5x + 6) = (negative) / (negative) = positive.
Mark the region as positive.
. . . .{+} . . . . (-19/15) . . . . . . . . . .(-18/15) . . . . . . . . . . . . . . .
For -19/15 < x < -18/15, it's a tricky area to test... if we make them both over 30, we get
-38/30 < x < -36/30
So let's test x = -37/30.
(15x + 19) / (5x + 6) = ( 15(-37/30) + 19) / (5(-37/30) + 6)
= (-37/2 + 19) / (-37/6 + 6)
= (-18.5 + 19) / (-37/6 + 36/6)
= (positive) / (negative)
= negative.
Mark the region as negative.
. . . .{+} . . . . (-19/15) . . . .{-} . . . .(-18/15) . . . . . . . . . . . . . . .
For x > -18/15, test x = 1000000000. Then
(15x + 19) / (5x + 6) = (positive) / (positive)
= positive.
Mark the region as positive.
. . . .{+} . . . . (-19/15) . . . .{-} . . . .(-18/15) . . . . .{+}. . . . . . . .
Look back at what the question is asking:
(15x + 19) / (5x + 6) > 0
So it is asking when the inequality is greater than 0, i.e. positive. Look at the regions themselves, it is positive when
(x < -19/15) OR (x > -18/15)
In interval notation, this is written as
(-infinity, -19/15) U (-18/15, infinity)
And that's your answer.