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If [z] denotes the least integer greater

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If [z] denotes the least integer greater [#permalink]

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New post 24 May 2017, 11:50
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Q.

If [z] denotes the least integer greater than or equal to z and [\(z^2\)] = 2, which of the following could be the value of [z]?

    I. 2
    II. 1
    III. -2


Answer Choices



    A. I only
    B. II only
    C. III only
    D. I and II only
    E. II and III only

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If [z] denotes the least integer greater [#permalink]

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New post 24 May 2017, 14:13
\([z^2] = 2\) means that

\(1<z^2\leq{2}\)

Now, Taking Square Root of both the sides.

If Z is positive, then

\(1<z\leq{\sqrt{2}}\)

then [z] = 2

If Z is negative, then

\(-1>z\geq{-\sqrt{2}}\)

then [z] = -1

Answer is A. I only
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Re: If [z] denotes the least integer greater than or equal to z and [z2] = [#permalink]

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New post 25 May 2017, 14:05
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1
niteshwaghray wrote:
If [z] denotes the least integer greater than or equal to z and [\(z^2\)] = 2, which of the following could be the value of [z]?

I) 2
II) 1
III) -2


A. I only
B. II only
C. III only
D. I and II only
E. II and III only


Given function rounds up a number to the nearest integer. For example, [1.3] = 2, because 2 is the least integer greater than or equal to 1.3.

[\(z^2\)] = 2;

\(1 < z^2 \leq 2\);

\(1 <|z| \leq \sqrt{2}\) (\(\sqrt{2} \approx 1.4\)).

\(1 < z \leq \sqrt{2}\) or \(-1 > z \geq \sqrt{2}\)

[z] can be 2 (if \(1 < z \leq \sqrt{2}\)) or -1 (if \(-1 > z \geq \sqrt{2}\)).

Answer: A.
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Re: If [z] denotes the least integer greater [#permalink]

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New post 31 May 2017, 06:09
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1

Official Solution



Given:

    • The function [z]
    • \([z^2] = 2\)

To find: Can [z] be {2, 1, -2}?

Approach:

    • To find which values of [z] are possible and which are not, we need to first know the range of possible values of z. Once we know what z can be, we’ll be able to find what [z] can be.

We’ll get an idea of the possible values of z from the fact that \([z^2] = 2\)

Working Out:

    • \([z^2] = 2\)

    • This means the least integer that is greater than or equal to \(z^2\) is 2.


    • We can write: \(1 < z^2 ≤ 2\)

    • The above inequality contains 2 inequalities: \(z^2 > 1\) AND \(z^2 ≤ 2\)
So now, we’ll solve these inequalities one by one, and then find the values of z that satisfy both these inequalities

    • Solving \(z^2 > 1\)

    • \(z^2 – 1 > 0\)

    • \((z+1)(z-1) > 0\)

This means, \(z < - 1\) or \(z > 1\) . . . (1)

    • Solving \(z^2 ≤ 2\)

    • \(z^2 – 2 ≤ 0\)

    • \((z + √2)(z-√2) ≤ 0\)

That is, \(-√2 ≤ z ≤ √2\) . . . (2)

Combining (1) and (2):

Combining them to find their overlap zones and finding those values of z that satisfy both the inequalities

    • So, either \(-√2 ≤ z < -1\) or \(1 < z ≤ √2\)

      • Either \(-√2 ≤ z < -1\)

      • \(-1.4  ≤ z < -1\)

    • In this case, \([z] = -1\)

      • Or \(1 < z ≤ √2\)

      • \(1 < z ≤ 1.4 \)

    • \([z] = 2\)
Thus, we see that [z] is either equal to -1 or equal to 2. Out of the 3 given values, only value I (which is 2) is therefore possible.

Looking at the answer choices, we see that the correct answer is Option A


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If [z] denotes the least integer greater than or equal to z and [z^2] [#permalink]

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New post 05 Mar 2018, 07:01
If [z] denotes the least integer greater than or equal to z and [z^2] = 2, which of the following could be the value of [z]?

I. 2
II. 1
III. -2

(A) I only

(B) II only

(C) III only

(D) I and II only

(E) II and III only
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Re: If [z] denotes the least integer greater than or equal to z and [z^2] [#permalink]

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New post 05 Mar 2018, 07:25
1
itisSheldon wrote:
If [z] denotes the least integer greater than or equal to z and [z^2] = 2, which of the following could be the value of [z]?

I. 2
II. 1
III. -2

(A) I only

(B) II only

(C) III only

(D) I and II only

(E) II and III only



Good question...
if [z^2] =2, \(z^2\) lies between 2 and 3
therefore z lies between \(\sqrt{2}=1.4\) and \(\sqrt{3}=1.7\) or between \(-\sqrt{2}=-1.4\) and \(-\sqrt{3}=-1.7\) if z is negative..
so when z is positive it lies between 1.4 and 1.7 and the NEXT greater integer is 2 so [z]=2
but when z is negative it lies between -1.4 and -1.7 and the NEXT greater integer is -1 so [z]=-1.. Be careful here and do not take -2

Only 2 is given
ans A
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Re: If [z] denotes the least integer greater [#permalink]

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New post 05 Mar 2018, 07:30
itisSheldon wrote:
If [z] denotes the least integer greater than or equal to z and [z^2] = 2, which of the following could be the value of [z]?

I. 2
II. 1
III. -2

(A) I only

(B) II only

(C) III only

(D) I and II only

(E) II and III only


Merging topics. Please search before posting. Thank you.
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Resources:
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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: If [z] denotes the least integer greater   [#permalink] 05 Mar 2018, 07:30
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