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Find all values of z such that \dfrac{3-z}{z+1} \ge 1. Answer in interval notation. 

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 Oct 25, 2014

Best Answer 

 #1
avatar+23254 
+6

Consider three cases:

1) z = -1: this is impossible because the denominator would become zero.

2) z > -1  (This makes the denominator positive; so when you multiply both sides by z + 1, you do not

                     need to change the direction of the inequality.)

     (3 - z) / (z + 1) ≥ 1

     Multiply both sides by z + 1:

      3 - z ≥ 1(z + 1)

      3 - z ≥ z + 1

      2 - z ≥ z

         2  ≥ 2z

         1 ≥ z     --->  z ≤ 1 and z > -1     --->     -1 < z ≤ 1

3)  z  < -1  (This makes the denominator negative; so when you multiply both sides by z + 1, you have

                     to change the direction of the inequality.)

     (3 - z) / (z + 1) ≥ 1

     Multiply both sides by z + 1:

      3 - z ≤ 1(z + 1)

      3 - z ≤  z + 1

      2 - z ≥  z

         2  ≤  2z

         1 ≤  z     --->  z ≥ 1  (This is imposible since the assumption of this section was z  < -1.)

Conclusion:  the answer is:  -1 < z ≤ 1      <==============               

 Oct 25, 2014
 #1
avatar+23254 
+6
Best Answer

Consider three cases:

1) z = -1: this is impossible because the denominator would become zero.

2) z > -1  (This makes the denominator positive; so when you multiply both sides by z + 1, you do not

                     need to change the direction of the inequality.)

     (3 - z) / (z + 1) ≥ 1

     Multiply both sides by z + 1:

      3 - z ≥ 1(z + 1)

      3 - z ≥ z + 1

      2 - z ≥ z

         2  ≥ 2z

         1 ≥ z     --->  z ≤ 1 and z > -1     --->     -1 < z ≤ 1

3)  z  < -1  (This makes the denominator negative; so when you multiply both sides by z + 1, you have

                     to change the direction of the inequality.)

     (3 - z) / (z + 1) ≥ 1

     Multiply both sides by z + 1:

      3 - z ≤ 1(z + 1)

      3 - z ≤  z + 1

      2 - z ≥  z

         2  ≤  2z

         1 ≤  z     --->  z ≥ 1  (This is imposible since the assumption of this section was z  < -1.)

Conclusion:  the answer is:  -1 < z ≤ 1      <==============               

geno3141 Oct 25, 2014

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