How do you solve #(4x-5)/(x+3)>0#?

1 Answer
May 10, 2015

#0 < (4x-5)/(x+3) = ((4x+12) - 17)/(x+3) = (4(x+3) -17)/(x+3) = 4 - 17/(x+3).#

Add #17/(x+3)# to both sides to get:

#17/(x+3) < 4#

There are now two permissible cases:

Case 1: When #x < -3#, #x + 3 < 0#, so if we multiply both sides by #(x+3)# we have to reverse the inequality to get:

#17>4(x+3)=4x+12#

Subtract 12 from both sides to get:

#5>4x#.

Divide both sides by 4 to get:

#5/4>x#, i.e. #x < 5/4#.

Since in this case we already know #x<-3#, this condition is already fulfilled.

Case 2: When #x > -3#, #x + 3 > 0#, so we can multiply both sides by #(x+3)# without reversing the inequality to get:

#17<4(x+3)=4x+12#

Subtract 12 from both sides to get:

#5<4x#.

Divide both sides by 4 to get:

#5/4<x#, i.e. #x > 5/4#.

If #x > 5/4# then it satisfies #x > -3#

So in Case 2, we just require #x > 5/4#.

Combining the 2 cases, we find that #x < -3# or #x > 5/4#.

Note that #x = -3# is not allowed due to the resulting division by 0, which is undefined.