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Q.

Find a number x   such that x 2   is an irrational number but x 4   is a rational number.


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a

3  

b

3 3  

c

3    

d

3 4  

answer is A.

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Detailed Solution

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To find such a number, we take a number whose fourth power makes the number rational but the second power does not.
From option (1),
Let’s assume x= 3 4 .  
We have,
x 4 = ( 3 4 ) 4 =3,   which is a rational number.
Also,
x 2 = ( 3 4 ) 2 = 3 ,   which is irrational.
Therefore, the value of x   could be 3 4 .   From option (2),
Let’s assume x= 3 .  
We have,
x 4 = 3 4 =9,   which is a rational number.
Also,
x 2 = 3 2 =3,   which is also a rational number.
Therefore, the value of x   couldn’t be 3 .   From option (3),
Let’s assume x= 3 3 .  
We have,
x 4 = ( 3 3 ) 4 =3 3 3 ,   which is an irrational number.
Also,
x 2 = ( 3 3 ) 2 = 3 2 3 ,   which is also irrational.
Therefore, the value of x   couldn’t be 3 3 .   From option (1),
Let’s assume x=3.  
We have,
x 4 = (3) 4 =81,   which is a rational number.
Also,
x 2 = (3) 2 =9,   which is also rational.
Therefore, the value of x   couldn’t be 3.
Hence, option (1) is correct.
 
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