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Question

If $y(x) = \begin{vmatrix} \sin x & \cos x & \sin x + \cos x + 1 \\ 27 & 28 & 27 \\ 1 & 1 & 1 \end{vmatrix}$, $x \in R$, then $\frac{d^2y}{dx^2} + y$ is equal to

The correct answer is

1

Determinant Function Derivative Calculation

The given function is defined by a determinant:

$ y(x) = \begin{vmatrix} \sin x & \cos x & \sin x + \cos x + 1 \\ 27 & 28 & 27 \\ 1 & 1 & 1 \end{vmatrix} $

Simplify Determinant

To simplify the determinant, apply the row operation $R_2 \to R_2 - 27 R_3$. This operation does not change the value of the determinant.

The second row \( [27, 28, 27] \) transforms as follows:

  • \( [27, 28, 27] - 27 \times [1, 1, 1] = [27-27, 28-27, 27-27] = [0, 1, 0] \)

The determinant simplifies to:

$ y(x) = \begin{vmatrix} \sin x & \cos x & \sin x + \cos x + 1 \\ 0 & 1 & 0 \\ 1 & 1 & 1 \end{vmatrix} $

Expand this determinant along the second row ($R_2$) for easier calculation:

$ y(x) = -1 \cdot \begin{vmatrix} \sin x & \sin x + \cos x + 1 \\ 1 & 1 \end{vmatrix} $

Calculate the remaining 2x2 determinant:

$ y(x) = -1 \left( (\sin x)(1) - (\sin x + \cos x + 1)(1) \right) $

$ y(x) = -1 \left( \sin x - \sin x - \cos x - 1 \right) $

$ y(x) = -1 \left( -\cos x - 1 \right) $

$ y(x) = \cos x + 1 $

Calculate Derivatives

Now, find the first and second derivatives of \( y(x) = \cos x + 1 \).

The first derivative is:

$ \frac{dy}{dx} = \frac{d}{dx}(\cos x + 1) = -\sin x $

The second derivative is:

$ \frac{d^2y}{dx^2} = \frac{d}{dx}(-\sin x) = -\cos x $

Final Expression Evaluation

Finally, evaluate the expression \( \frac{d^2y}{dx^2} + y \):

$ \frac{d^2y}{dx^2} + y = (-\cos x) + (\cos x + 1) $

Combine like terms:

$ \frac{d^2y}{dx^2} + y = -\cos x + \cos x + 1 $

$ \frac{d^2y}{dx^2} + y = 1 $

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