Question Details

Let S be the set of all  ( α , β ) R × R such that

lim x sin ( x 2 ) ( log e x ) α sin ( 1 x 2 ) x α β ( log e ( 1 + x ) ) β = 0

Then which of the following is (are) correct?

Options

A

( 1 , 3 ) S

B

( 1 , 1 ) S

C

( 1 , 1 ) S

D

( 1 , 2 ) S

Show Answer

Correct Answer :

Option B

( 1 , 1 ) S

(-1, 1) \in S

Solution :

The correct answer is:
( 1 , 1 ) S

To evaluate the limit and determine the conditions on α and β, we analyze the behavior of the terms in the limit expression as x.

Let the expression in the limit be L:
L = sin ( x 2 ) ( log e x ) α sin ( 1 x 2 ) x α β ( log e ( 1 + x ) ) β

We use the following asymptotic behaviors as x:
1. Since 1x20, we have:
sin ( 1 x 2 ) 1 x 2
2. For the logarithmic term in the denominator:
log e ( 1 + x ) = log e [ x ( 1 + 1 x ) ] = log e x + log e ( 1 + 1 x ) log e x

Substituting these into the limit expression gives the simplified form:
L sin ( x 2 ) ( log x ) α 1 x 2 x α β ( log x ) β = sin ( x 2 ) x ( 2 + α β ) ( log x ) α β

Since sin(x2) is bounded, the limit as x will be 0 if the power of x dominates and drives the expression to zero. Specifically, if 2+αβ>0, then x(2+αβ)0, and because powers of x dominate over logarithmic terms, the limit is 0.

Let us test the option (α,β)=(1,1):
2 + α β = 2 + ( 1 ) ( 1 ) = 1 > 0

Evaluating the limit for these values:
lim x sin ( x 2 ) x ( log x ) 2 = 0
This confirms that (1,1)S.

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