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Inequality and absolute value questions from my collection [#permalink]
16 Nov 2009, 10:33

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Guys I didn't forget your request, just was collecting good questions to post.

So here are some inequality and absolute value questions from my collection. Not every problem below is hard, but there are a few, which are quite tricky. Please provide your explanations along with the answers.

1. If 6*x*y = x^2*y + 9*y, what is the value of xy? (1) y – x = 3 (2) x^3< 0

Re: Inequality and absolute value questions from my collection [#permalink]
17 Nov 2009, 09:54

ichha148 wrote:

12. Is r=s?

(1) -s<=r<=s

(2) |r|>=s

E – for this - both can be true or false when 0< r < 1 For example , take r as 0.8 S = 0.86 i.e. -0.86 < = 0.8 < = 0.86 |0.8|>= 0.86 i.e. 1 >= 0.86 Combining , any values can be taken , on values > =1 , both r and s will be same

Taking the modulus does not mean rounding up to the nearest integer; it means removing the negative sign if present. |0.8|<0.86

ichha148 wrote:

3. Is x^2 + y^2 > 4a?

(1) (x + y)^2 = 9a

(2) (x – y)^2 = a C is the answer

Combined both and the equation will give x^2 + y^2 = 5a

Nowhere it is said that x and y are non-zero. If x and y are zero, 5a=0, therefore a=0, and the stem is false (x^2+y^2=0)

Last edited by Marco83 on 17 Nov 2009, 09:57, edited 1 time in total.

Re: Inequality and absolute value questions from my collection [#permalink]
17 Nov 2009, 13:01

Quote:

4. Are x and y both positive? (1) 2x-2y=1 (2) x/y>1

Question Stem : x > 0 ; y > 0 ?

St. (1) : 2x -2y = 1 x = y + 0.5 Equation can be satisfied for both positive and negative values of x and y. Hence Insufficient.

St. (2) : x/y > 1 Equation can be satisfied when both x and y are either positive or negative. Hence Insufficient.

St. (1) and (2) together : (y + 0.5)/y > 1 1 + 0.5/y > 1 0.5/y > 1 For this to be true, y must be positive. If y is positive then x will also be positive. Hence Sufficient.

Re: Inequality and absolute value questions from my collection [#permalink]
17 Nov 2009, 13:10

Quote:

5. What is the value of y? (1) 3|x^2 -4| = y - 2 (2) |3 - y| = 11

Question stem : What is the exact value of y?

St. (1) : 3*|x^2 -4| = y - 2 y = 3*|x^2 -4| + 2 From this we can infer that y will be a positive value. That is, y > 0. However, we want to know the exact value of y. Therefore, Insufficient.

St. (2) : |3 - y| = 11 (a) When (3 - y) > 0 ; 3 - y = 11 ; y = -8. (b) When (3 - y) < 0 ; - (3 - y) = 11 ; y = 14. Thus we can see that there are two possible values for y. Hence Insufficient.

St. (1) and (2) together : y > 0 ; y = 14 or -8. Obviously since y has to be greater than 0, it cannot be -8. Therefore value of y = 14. Hence Sufficient.

Re: Inequality and absolute value questions from my collection [#permalink]
17 Nov 2009, 13:16

Quote:

6. If x and y are integer, is y > 0? (1) x +1 > 0 (2) xy > 0

St. (1) : x + 1 > 0 Tells us nothing about y. Insufficient.

St. (2) : xy > 0 Both x and y can either be positive or negative. Neither x nor y can be 0. Insufficient.

St. (1) and (2) together : Since x is an integer and cannot hold the value 0, it has to be greater than 1 in order to satisfy St. (1). Since we know that x will be positive, y will also have to be a positive integer in order to satisfy St. (2). Hence Sufficient.

Re: Inequality and absolute value questions from my collection [#permalink]
17 Nov 2009, 13:43

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Quote:

7. |x+2|=|y+2| what is the value of x+y? (1) xy<0 (2) x>2 y<2

Question stem : Note: Since the equations is symmetrical, there will only be two distinct cases. However, for the sake of explanation, I have illustrated all 4. (a) When both x and y are greater than - 2 ; x + 2 = y + 2 ; x = y (b) When both x and y are less than - 2 ; - x - 2 = - y - 2 ; x = y (c) When x is less than -2 and y is greater than -2 ; - x - 2 = y + 2 ; x + y = - 4 (d) When x is greater than -2 and y is less than -2 ; x + 2 = - y - 2 ; x + y = - 4

St. (1) : xy < 0 This implies that one is negative and the other is positive. Therefore, in order for xy to be less than 0, x cannot be equal to y. Thus in order to satisfy the question stem, it can only be cases (c) and (d). Thus Sufficient.

St. (2) : x > 2 ; y < 2 Again, this implies that x and y cannot be equal. Thus, in order to satisfy the question stem it can only be cases (c) and (d). Thus Sufficient.

Re: Inequality and absolute value questions from my collection [#permalink]
17 Nov 2009, 13:54

Quote:

9. Is n<0? (1) -n=|-n| (2) n^2=16

Question Stem : Is n negative?

St. (1) : -n = |-n| Let -n = A ; therefore the statement becomes : A = |A|. This can only be valid when A is positive (or equal to 0). This in turn means that n must be negative (or equal to 0). -n=|-n| also for n=0, hence not sufficient.

Re: Inequality and absolute value questions from my collection [#permalink]
17 Nov 2009, 14:29

Quote:

11. Is |x+y|>|x-y|? (1) |x| > |y| (2) |x-y| < |x|

Question Stem : Is the distance between X and -Y greater than the distance between X and Y? Note : Using number line approach.

St. (1) : |x| > |y| OR, The distance between X and the origin is greater than the distance between Y and the origin.

Now we can have two cases :

(a) When X is positive : In this case, X > Y for the above condition to be true.

_________|_________|_________|_________|_________|__________ _________________-Y______Origin______Y........ Region of X.........

Thus we can see that the distance between X and - Y will always be greater than the distance between X and Y. Hence question stem is true.

(b) When X is negative : In this case, X < -Y for the statement to be true.

_________|_________|_________|_________|_________|__________ .... Region of X.....-Y_____Origin_______Y___________

Thus we can see that the distance between X and -Y will always be less than the distance between X and Y. Hence, question stem becomes false.

Due to conflicting statements, St (1) becomes Insufficient.

St. (2) : |x-y| < |x| OR, the distance between X and Y is less than the distance between X and the origin.

Now again let us consider the different cases :

(a) When X is positive : For the statement to be true and for X to be positive, X must be greater than Y/2. For any value of X less than Y/2 the statement will become false. The statement will be true for any value greater than Y/2.

Thus we see that only one case comes into the picture. Now let us see how it relates to the question stem.

_________|_________|____;____|_________|________ ________-Y____Origin__y/2...Y...... Region of X....

Thus we can see that the distance between X and - Y will be greater than the distance between X and Y for all values of X > Y/2. Thus question stem is true. St. (2) is sufficient.

Question Stem : Is |x-1| < 1 ? When x > 1 ; x - 1 < 1 ; x < 2. When x < 1 ; -x + 1 < 1 ; x > 0. Thus it can be written as : 0 < x < 2.

St. (1) : (x-1)^2 <= 1 x^2 + 1 - 2x <= 1 x^2 - 2x <= 0 x(x - 2) <= 0 ; Thus boundary values are 0 and 2. Therefore statement can be written as : 0 <= x <= 2. Since the values are inclusive of 0 and 2, it cannot give us the answer. Insufficient.

St. (2) : x^2 - 1 > 0 (x + 1)*(x - 1) > 0 Statement can be written as x > 1 and x < -1. Thus it is possible for x to hold values which make the question stem true as well as false. Insufficient.

St. (1) and (2) : 0 <= x <= 2 ; x > 1 and x < -1 Thus combined, the statements become : 1 < x <= 2. Since it is inclusive of 2, it will give us conflicting solutions for the question stem. Hence Insufficient.

Re: Inequality and absolute value questions from my collection [#permalink]
17 Nov 2009, 17:24

sriharimurthy wrote:

Quote:

9. Is n<0? (1) -n=|-n| (2) n^2=16

Question Stem : Is n negative?

St. (1) : -n = |-n| Let -n = A ; therefore the statement becomes : A = |A|. This can only be valid when A is positive. This in turn means that n must be negative. Thus Sufficient.

Re: Inequality and absolute value questions from my collection [#permalink]
18 Nov 2009, 00:01

Marco83 wrote:

sriharimurthy wrote:

Quote:

9. Is n<0? (1) -n=|-n| (2) n^2=16

Question Stem : Is n negative?

St. (1) : -n = |-n| Let -n = A ; therefore the statement becomes : A = |A|. This can only be valid when A is positive. This in turn means that n must be negative. Thus Sufficient.

-n=|-n| also for n=0, hence not sufficient.

Everything else is as you suggested, therefore C

Yes, you are right. I overlooked that. Thanks. _________________

Re: Inequality and absolute value questions from my collection [#permalink]
18 Nov 2009, 07:39

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SOLUTIONS:

1. If 6*x*y = x^2*y + 9*y, what is the value of xy? (1) y – x = 3 (2) x^3< 0

First let's simplify given expression \(6*x*y = x^2*y + 9*y\):

\(y*(x^2-6x+9)=0\) --> \(y*(x-3)^2=0\). Note here that we CAN NOT reduce this expression by \(y\), as some of you did. Remember we are asked to determine the value of \(xy\), and when reducing by \(y\) you are assuming that \(y\) doesn't equal to \(0\). We don't know that.

Next: we can conclude that either \(x=3\) or/and \(y=0\). Which means that \(xy\) equals to 0, when y=0 and x any value (including 3), OR \(xy=3*y\) when y is not equal to zero, and x=3.

(1) \(y-x=3\). If y is not 0, x must be 3 and y-x to be 3, y must be 6. In this case \(xy=18\). But if y=0 then x=-3 and \(xy=0\). Two possible scenarios. Not sufficient.

OR:

\(y-x=3\) --> \(x=y-3\) --> \(y*(x-3)^2=y*(y-3-3)^2=y(y-6)^2=0\) --> either \(y=0\) or \(y=6\) --> if \(y=0\), then \(x=-3\) and \(xy=0\) \(or\) if \(y=6\), then \(x=3\) and \(xy=18\). Two different answers. Not sufficient.

(2) \(x^3<0\). x is negative, hence x is not equals to 3, hence y must be 0. So, xy=0. Sufficient.

Answer: B.

This one was quite tricky and was solved incorrectly by all of you.

Never reduce equation by variable (or expression with variable), if you are not certain that variable (or expression with variable) doesn't equal to zero. We can not divide by zero.

Never multiply (or reduce) inequality by variable (or expression with variable) if you don't know the sign of it or are not certain that variable (or expression with variable) doesn't equal to zero. _________________

Re: Inequality and absolute value questions from my collection [#permalink]
18 Nov 2009, 07:47

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2. If y is an integer and y = |x| + x, is y = 0? (1) x < 0 (2) y < 1

Note: as \(y=|x|+x\) then \(y\) is never negative. For \(x>{0}\) then \(y=x+x=2x\) and for \(x\leq{0}\) then (when x is negative or zero) then \(y=-x+x=0\).

(1) \(x<0\) --> \(y=|x|+x=-x+x=0\). Sufficient.

(2) \(y<1\), as we concluded y is never negative, and we are given that \(y\) is an integer, hence \(y=0\). Sufficient.

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