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If z^2 – 4z > 5, which of the following is always true?

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Joined: 26 Feb 2017
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GMAT 1: 610 Q45 V28
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If z^2 – 4z > 5, which of the following is always true?  [#permalink]

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New post 23 Oct 2017, 19:07
4
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A
B
C
D
E

Difficulty:

  85% (hard)

Question Stats:

38% (01:26) correct 62% (01:34) wrong based on 155 sessions

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If z^2 – 4z > 5, which of the following is always true?

(A) z > 5
(B) z < 5
(C) z > -1
(D) z < 1
(E) none of above
Math Expert
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Joined: 02 Aug 2009
Posts: 6979
If z^2 – 4z > 5, which of the following is always true?  [#permalink]

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New post 23 Oct 2017, 19:44
1
2
vitorpteixeira wrote:
If z^2 – 4z > 5, which of the following is always true?
(A) z > 5
(B) z < 5
(C)z > -1
(D)z < 1
(E) none of above



lets solve the equation..
\(z^2-4z>5........z^2-4z-5>0.....(z-5)(z+1)>0\)

so two options ..
1) BOTH z-5 and z+1 are positive..
when z>5 both are positive
2) both are negative
z-5 and z+1..
z<-1, both are negative

so z<-1 and or z>5
two ranges possible
Insufficient..

E
_________________

1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html


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If z^2 – 4z > 5, which of the following is always true?  [#permalink]

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New post 23 Oct 2017, 21:03
1
Hi, answer must be E.

As z^2 - 4z > 5
=> z^2 - 4z - 5 > 0
(z - 5)(z + 1) > 0

when -1 < z < 5, (z-5)(z+1) < 0 always
when z = -1 or 5, (z-5)(z+1) = 0
when z < -1 or z > 5, then (z-5)(z+1) > 0 always

since none of the options match z < -1 or z > 5, answer must be E
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Re: If z^2 – 4z > 5, which of the following is always true?  [#permalink]

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New post 24 Oct 2017, 03:50
chetan2u wrote:
vitorpteixeira wrote:
If z^2 – 4z > 5, which of the following is always true?
(A) z > 5
(B) z < 5
(C)z > -1
(D)z < 1
(E) none of above



lets solve the equation..
\(z^2-4z>5........z^2-4z-5>0.....(z-5)(z+1)>0\)

so two options ..
1) BOTH z-5 and z+1 are positive..
when z>5 both are positive
2) both are negative
z-5 and z+1..
z<-1, both are negative

so z<-1 and or z>5
two ranges possible
Insufficient..

E



My doubt is, since any value Z>5 satisfy the inequality, why A is wrong?
Math Expert
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V
Joined: 02 Aug 2009
Posts: 6979
If z^2 – 4z > 5, which of the following is always true?  [#permalink]

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New post 24 Oct 2017, 06:15
2
1
vitorpteixeira wrote:
chetan2u wrote:
vitorpteixeira wrote:
If z^2 – 4z > 5, which of the following is always true?
(A) z > 5
(B) z < 5
(C)z > -1
(D)z < 1
(E) none of above



lets solve the equation..
\(z^2-4z>5........z^2-4z-5>0.....(z-5)(z+1)>0\)

so two options ..
1) BOTH z-5 and z+1 are positive..
when z>5 both are positive
2) both are negative
z-5 and z+1..
z<-1, both are negative

so z<-1 and or z>5
two ranges possible
Insufficient..

E



My doubt is, since any value Z>5 satisfy the inequality, why A is wrong?



Yes, the answer would be A if the Q asked CAN be true..
But Question is ALWAYS true.
So the equation is true when x<-1..
This means z can be -2, so z need not ALWAYS be >5

Hope it helps
_________________

1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html


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Re: If z^2 – 4z > 5, which of the following is always true?  [#permalink]

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New post 26 Oct 2017, 15:39
vitorpteixeira wrote:
If z^2 – 4z > 5, which of the following is always true?

(A) z > 5
(B) z < 5
(C) z > -1
(D) z < 1
(E) none of above


We’ll treat it as an equation first:

z^2 – 4z = 5

z^2 – 4z – 5 = 0

(z - 5)(z + 1) = 0

z = 5 or z = -1

The two solutions above divide the number line into three intervals:

1) z < -1

2) -1 < z < 5

3) z > 5

For each of these intervals, if one number from the interval satisfies the inequality, then the whole interval will satisfy the inequality.

1) z < -1

Let’s pick z = -2:

(-2)^2 - 4(-2) > 5?

4 + 8 > 5? (Yes)

2) -1 < z < 5

Let’s pick z = 0:

0^2 - 4(0) > 5?

0 - 0 > 5? (No)

3) z > 5

Let’s pick z = 6:

6^2 - 4(6) > 5?

36 - 24 > 5? (Yes)

We see that the solution to the inequality is z < -1 or z > 5. Note that the question is asking for a statement that is ALWAYS true; for any of the given answer choices, we can always find a value of z that does not satisfy the given inequality. Thus, none of the provided answer choices are always true.

Answer: E
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Re: If z^2 – 4z > 5, which of the following is always true? &nbs [#permalink] 26 Oct 2017, 15:39
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