Question Details

Consider the equation 4√2x3 − 3√2x −1 = 0 .

Statement 1: Solution of this equation is cos(π/12) .

Statement 2: This equation has only one real solution.

Options

A

Both statement 1 and statement 2 are true

B

Statement 1 is true but statement 2 is false

C

Statement 1 is false but statement 2 is true

D

Both statement 1 and statement 2 are false

Correct Answer :

Statement 1 is true but statement 2 is false

Solution :

To analyze the given statements, we start with the cubic equation:

4 2 x 3 - 3 2 x - 1 = 0

First, we can divide the entire equation by 2 (since 20):

4 x 3 - 3 x - 1 2 = 0

This can be rewritten as:

4 x 3 - 3 x = 1 2

Recall the trigonometric triple-angle identity for cosine:

cos ( 3 θ ) = 4 cos 3 ( θ ) - 3 cos ( θ )

Let us substitute x=cos(θ) into our equation, where we restrict θ[0,π] to find unique real roots:

cos ( 3 θ ) = 1 2

Since θ[0,π], the angle 3θ lies in the interval [0,3π]. Within this interval, we solve for 3θ where the cosine value is 12:

1) 3 θ = π 4 θ = π 12
2) 3 θ = 2 π - π 4 = 7 π 4 θ = 7 π 12
3) 3 θ = 2 π + π 4 = 9 π 4 θ = 9 π 12 = 3 π 4

Each of these three values of θ lies in the range [0,π] and yields a distinct value for x=cos(θ):

x 1 = cos π 12
x 2 = cos 7 π 12
x 3 = cos 3 π 4

Thus, we have three distinct real solutions. Let us evaluate the statements:

Statement 1: "Solution of this equation is cos(π/12)". Since x1=cos(π/12) is indeed a root of the equation, Statement 1 is true.

Statement 2: "This equation has only one real solution". Since we found three distinct real solutions, Statement 2 is false.

Therefore, Statement 1 is true but Statement 2 is false.

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