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Re: As x increases from 165 to 166, which of the following must [#permalink]

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13 Aug 2013, 08:12

Walkabout wrote:

As x increases from 165 to 166, which of the following must increase?

I. 2x - 5 II. 1 - 1/x III. 1/(x^2 - x)

(A) I only (B) III only (C) I and II (D) I and III (E) II and III

... I and II can be counted easily, both increases. And III is 1/x(x-1) Now, 1/(165×164) > 1/(166×165) , so it decreases as 165 turns to 166 . so I and II = C (Answer) _________________

Re: As x increases from 165 to 166, which of the following must [#permalink]

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13 Aug 2013, 10:40

Walkabout wrote:

As x increases from 165 to 166, which of the following must increase?

I. 2x - 5 II. 1 - 1/x III. 1/(x^2 - x)

(A) I only (B) III only (C) I and II (D) I and III (E) II and III

The max difference b/w 165 and 166 is '1' and the min value can be 0.000000000000000000001 but for our convenience we shall choose 0.1 Equation 1 ==> for min value of 0.1, the value is 2(0.1) - 5 = -4.8

for max value of 1, 2(1)-5=-3 so the values are increasing from -4.8 to -3

similary submit the values 0.1 and 1 and we notice that the value increases from -9 to 0 ==> equation 2 is also increasing

for double confirmation substitute the values of 0.1 and 1 in equation 3. for min value of x=0.1, the value is \(1/(0.01-0.1)=-1/0.09\) for max value of 1, the value is 'infinite' ... hence we dont count this.

So equations 1 & 2 are increasing. This is the answer _________________

As x increases from 165 to 166, which of the following must [#permalink]

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22 Oct 2014, 07:26

selfishmofo wrote:

For the first part, I simply plugged in a positive whole number for x, then I retested with a higher number than the number I chose originally for x, ..... ...... For the second problem, I used the same method, third problem, decreases by theory, no need to solve.

I have done the first two in the same way you did, but third one took time. Can you please connect or explain the theory that helped you?

Re: As x increases from 165 to 166, which of the following must [#permalink]

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22 Oct 2014, 07:49

1

This post received KUDOS

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appleid wrote:

selfishmofo wrote:

For the first part, I simply plugged in a positive whole number for x, then I retested with a higher number than the number I chose originally for x, ..... ...... For the second problem, I used the same method, third problem, decreases by theory, no need to solve.

I have done the first two in the same way you did, but third one took time. Can you please connect or explain the theory that helped you?

x^2 - x is an equation of upward parabola, intercepting the x axis at 0 and 1.

Attachment:

graph.png [ 7.08 KiB | Viewed 5131 times ]

So, far away from the roots, the values of the function (x^2 - x) are increasing as x increases. Therefore 1/(x^2 - x) decreases.

As x increases from 165 to 166, which of the following must [#permalink]

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24 Oct 2014, 04:11

Bunuel wrote:

appleid wrote:

selfishmofo wrote:

For the first part, I simply plugged in a positive whole number for x, then I retested with a higher number than the number I chose originally for x, ..... ...... For the second problem, I used the same method, third problem, decreases by theory, no need to solve.

I have done the first two in the same way you did, but third one took time. Can you please connect or explain the theory that helped you?

x^2 - x is an equation of upward parabola, intercepting the x axis at 0 and 1.

Attachment:

graph.png

So, far away from the roots, the values of the function (x^2 - x) are increasing as x increases. Therefore 1/(x^2 - x) decreases.

Similar questions to practice: .... Hope it helps.

Thank you! However, I did understand the value of x^2-x will always goes up, but I am not getting the basic theory (or relevant foundation/topic) behind it. Could you please suggest me any topic which will teach me this from ground level?

Re: As x increases from 165 to 166, which of the following must [#permalink]

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25 Nov 2015, 17:10

Bunuel wrote:

Walkabout wrote:

As x increases from 165 to 166, which of the following must increase?

I. 2x - 5 II. 1 - 1/x III. 1/(x^2 - x)

(A) I only (B) III only (C) I and II (D) I and III (E) II and III

I. 2x - 5 --> x increases from 165 to 166 --> 2x increases --> 2x - 5 increases. Correct.

II. 1 - 1/x --> x increases from 165 to 166 --> 1/x decreases --> 1 -1/x increases. Correct.

III. 1/(x^2 - x) --> x increases from 165 to 166 --> x^2-x increases --> 1/(x^2 - x) decreases.

Answer: C.

Hi Bunuel, I was thinking that plugging in 2 instead of 165 and 3 instead of 166 and then solve could be an efficient approach for this particular question. Do you agree? _________________

Consider giving me Kudos if I helped, but don´t take them away if I didn´t!

Re: As x increases from 165 to 166, which of the following must [#permalink]

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28 Nov 2015, 21:05

2

This post received KUDOS

Expert's post

Hi All,

Roman Numeral questions are relatively rare on Test Day (you'll likely see just 1 in the Quant section), but they are almost always designed around Number Properties and logic 'shortcuts'...

Here, we're told that X increase from 165 to 166. We're asked which of the following MUST increase. While this prompt might appear 'calculation heavy', you can actually solve it WITHOUT doing calculations (and paying attention to the answer choices).

I. 2X - 5

The "-5" has the same 'math effect', regardless of what X is: the '-5' reduces the value by 5. So as X goes from 165 to 166, 2X gets BIGGER. Since the '-5' has the same effect, 2X-5 definitely gets bigger as X increases from 165 to 166. Roman Numeral 1 INCREASES Eliminate Answers B and E.

II. 1 - 1/X

As the denominator of a fraction gets BIGGER, the fraction gets SMALLER. Thus, 1/165 > 1/166. The "1" in this calculation is constant, so subtracting a SMALLER fraction from it will lead to a BIGGER result. As X increases from 165 to 166, 1/X gets SMALLER, so 1-(1/X) definitely gets bigger. Roman Numeral 2 INCREASES Eliminate Answers A and D.

There's only one answer remaining (and we don't even have to deal with Roman Numeral 3)...

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