Question Details

The limit  p = lim x π ( x 2 + α x + 2 π 2 x π + 2 sin x ) has a finite value for a real α. The value of α and the corresponding limit p are

Options

A

α = −3π, and p = π

B

α = −2π, and p = 2π

C

α = π, and p = π

D

α = 2π, and p = 3π

Correct Answer :

α = −3π, and p = π

Solution :

The correct option is: α = −3π, and p = π

Let us analyze the limit:

p = lim x π ( x 2 + α x + 2 π 2 x π + 2 sin x )

First, we examine the behavior of the denominator as xπ:
lim x π ( x π + 2 sin x ) = π π + 2 sin ( π ) = 0

Since the limit p is given to be finite, the numerator must also approach 0 as xπ. If the numerator approached a non-zero constant, the limit would be undefined (tending to infinity).

Therefore, we set the limit of the numerator to 0:
lim x π ( x 2 + α x + 2 π 2 ) = 0

Substituting x=π into the numerator expression:
π 2 + α π + 2 π 2 = 0

Simplifying the terms:
3 π 2 + α π = 0
α π = 3 π 2
α = 3 π

Now, we substitute α=3π back into the original limit expression to evaluate p:
p = lim x π ( x 2 3 π x + 2 π 2 x π + 2 sin x )

This limit represents an indeterminate form of type 00. Therefore, we can apply L'Hôpital's Rule, which states that we can differentiate the numerator and denominator with respect to x to find the limit:

Differentiating the numerator:
d d x ( x 2 3 π x + 2 π 2 ) = 2 x 3 π

Differentiating the denominator:
d d x ( x π + 2 sin x ) = 1 + 2 cos x

Now, apply L'Hôpital's Rule:
p = lim x π 2 x 3 π 1 + 2 cos x

Evaluating the limit by substituting x=π:
p = 2 ( π ) 3 π < 1 + 2 cos ( π )

Since cos(π)=1:
p = 2 π 3 π < 1 + 2 ( 1 )
p = π 1 2
p = π 1 = π

Thus, we have successfully determined that:
α = −3π, and p = π

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