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Bunuel
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Rebaz
Bunuel

This is what i donot understand.

You say that mn must be more than −4∗7=−28, meaning mn >-28 or -28<mn

and that mn must be less than −9∗11=−99, meaning mn<-99 or -99>mn.

Then you go on and say: −99<mn<−28. How is this possible?

How can we go from mn>-28 to mn<-28 and mn <-99 to -99<mn?

What is it that i donot see?

Can you please elaborate more on the above?


Thanks in advance!

Multiply \(-99 < mn < -28\) by -1 and flip the signs:

\(99 > -mn > 28\), which is the same as \(28 < -mn < 99\).
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Bunuel,

Now i understand where i did go wrong, Thanks a lot!
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I think this is a high-quality question and I don't agree with the explanation. From the 1st statement - say m is -8 and from the second statement say n is 8. solving this for the equation will give 4 as a valid answer - how is this wrong?
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I think this is a high-quality question and I don't agree with the explanation. From the 1st statement - say m is -8 and from the second statement say n is 8. solving this for the equation will give 4 as a valid answer - how is this wrong?

If m = 8 and n = -8, \(\sqrt[4]{-mn}= \sqrt[4]{64}=2\sqrt{2} \), not 4.
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Can you please help me out why are you taking

- 9 * 11 < mn < -4 * 7

Why not -9 * 7 < mn < -4 * 11
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Pragun1710

Bunuel
Official Solution:


If \(-9 < m < -4\) and \(7 < n < 11\), which of the following can be a value of \(\sqrt[4]{-mn}\)?

I. 2

II. 3

III. 4


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


\(mn\) must be more than \(-4*7 = -28\) and less than \(-9*11 = -99\):

\(-99 < mn < -28\)

Multiplying by -1 gives:

\(28 < -mn < 99\)

Taking the fourth root gives:

\(\sqrt[4]{28} < \sqrt[4]{-mn} < \sqrt[4]{99}\)

Now, considering that \(2^4 < 28 < 3^4 < 99 < 4^4\), it follows that \(\sqrt[4]{-mn}\) can only be 3.


Answer: B

Can you please help me out why are you taking

- 9 * 11 < mn < -4 * 7

Why not -9 * 7 < mn < -4 * 11

With the method shown in the solution, we are getting the maximum range for mn. Notice that:

  • (-9 * 11 = -99) < (-9 * 7 = -63), and
  • (-4 * 7 = -28) > (-4 * 11 = -44).
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I did not quite understand the solution. if its solved like: m= -5,-6,-7,-8
n= 8,9,10
then mn=81 is not getting satisfied with the above. Can you please explain
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Aishna1034

Bunuel
Official Solution:


If \(-9 < m < -4\) and \(7 < n < 11\), which of the following can be a value of \(\sqrt[4]{-mn}\)?

I. 2

II. 3

III. 4


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


\(mn\) must be more than \(-4*7 = -28\) and less than \(-9*11 = -99\):

\(-99 < mn < -28\)

Multiplying by -1 gives:

\(28 < -mn < 99\)

Taking the fourth root gives:

\(\sqrt[4]{28} < \sqrt[4]{-mn} < \sqrt[4]{99}\)

Now, considering that \(2^4 < 28 < 3^4 < 99 < 4^4\), it follows that \(\sqrt[4]{-mn}\) can only be 3.


Answer: B
I did not quite understand the solution. if its solved like: m= -5,-6,-7,-8
n= 8,9,10
then mn=81 is not getting satisfied with the above. Can you please explain
m is not limited to integers like -5, -6, -7, -8. It can be any real number between -9 and -4. Similarly, n can be any real number between 7 and 11.

So you cannot just pick integer values. You have to treat m and n as continuous real numbers, and the correct range for -mn is 28 < -mn < 99 as shown.

Your method of picking integers does not apply here.
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I did not quite understand the solution. how do we infer this : mn must be less than −4∗7=−28 and more than −9∗11=−99 ?
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shumalik
I did not quite understand the solution. how do we infer this : mn must be less than −4∗7=−28 and more than −9∗11=−99 ?

Because m is always negative and n is always positive, the product mn is always negative. The smallest possible value of mn comes from multiplying the smallest m with the largest n (-9 * 11 = -99), and the largest possible value of mn comes from multiplying the largest m with the smallest n (-4 * 7 = -28). Hence, -99 < mn < -28.
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