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Solving an inequality

by Shaybay92
Tags: inequality, solving
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Shaybay92
#1
Mar14-10, 12:24 AM
P: 122
I am trying to find the domain of a square root function... To do so I have to solve the following inequality:


1/(x+1) - 4/(x-2) >= 0

This is how i attempted to solve it...:

I crossmultilplied the denominator to get

[(x-2) - 4(x+1)]/(x-2)(x+1) >= 0

Multiplied both sides by (x-2)(x+1)

(x-2) - 4(x+1) > = 0

Expanded

x - 2 -4x - 4 = 0

-3x -6 >= 0

-3(x+2) >= 0

(x+2) <= 0 <---- at this point I am not sure if i swap the sign around, I haven't been taught inequalities before... but I will swap it around anyway.

x <= -2

Is this the correct answer? When I graph the entire function (sqrt of the above), I get part of the function less than -2 but also part greater than -2.... I dont really understand how there can be x > -2 if I got this restriction here of >-2.
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rock.freak667
#2
Mar14-10, 12:36 AM
HW Helper
P: 6,202
When dealing with an inequality, if you multiply by a negative number, the inequality changes.

You can deal with this by multiplying by the square of the denominator

i.e. ((x+1)(x-2))2
BL4CKCR4Y0NS
#3
Mar17-10, 01:39 AM
P: 61
Yeah, when I calculate it I get the same answer:

x <= (-2)

Mark44
#4
Mar17-10, 09:39 AM
Mentor
P: 21,304
Solving an inequality

Just as rock.freak667 said, multiply both sides by (x+1)2(x-2)2. The domain is not just x <= -2.


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