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.