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Second Order Derivative

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Estimated time: 7 minutes
CBSE: Class 12

Definition: Second Order Derivative

Let y = f(x). If the first derivative \[\frac{dy}{dx} = f'(x)\] is itself differentiable, then differentiating once again with respect to \[x\] gives the second order derivative.

CBSE: Class 12

Example 1

If \[y = \text{A} \sin x + \text{B} \cos x\], then prove that \[\frac{d^{2}y}{dx^{2}} + y = 0\].

Solution: We have

\[\frac{dy}{dx} = \text{A} \cos x - \text{B} \sin x\]

and

\[\frac{d^{2}y}{dx^{2}} = \frac{d}{dx} (\text{A} \cos x - \text{B} \sin x)\]
\[= -\text{A} \sin x - \text{B} \cos x = -y\]

Hence \[\frac{d^{2}y}{dx^{2}} + y = 0\]

CBSE: Class 12

Example 2

If \[y = \sin^{-1} x\], show that \[(1 - x^2) \frac{d^2 y}{dx^2} - x \frac{dy}{dx} = 0\].

Solution: We have \[y = \sin^{-1} x\]. Then

\[\frac{dy}{dx} = \frac{1}{\sqrt{(1 - x^2)}}\]

or 

\[\sqrt{(1 - x^2)} \frac{dy}{dx} = 1\]

So

\[\frac{d}{dx} \left( \sqrt{(1 - x^2)} \cdot \frac{dy}{dx} \right) = 0\]

or

\[\sqrt{(1 - x^2)} \cdot \frac{d^2 y}{dx^2} + \frac{dy}{dx} \cdot \frac{d}{dx} (\sqrt{(1 - x^2)}) = 0\]

or

\[\sqrt{(1 - x^2)} \cdot \frac{d^2 y}{dx^2} - \frac{dy}{dx} \cdot \frac{2x}{2\sqrt{1 - x^2}} = 0\]

Hence \[(1 - x^2) \frac{d^2 y}{dx^2} - x \frac{dy}{dx} = 0\]

Alternatively, Given that \[y = \sin^{-1} x\], we have

\[y_1 = \frac{1}{\sqrt{1 - x^2}}, \text{ i.e., } (1 - x^2) y_1^2 = 1\]

So\[(1 - x^2) \cdot 2y_1 y_2 + y_1^2 (0 - 2x) = 0\]

Hence \[(1 - x^2) y_2 - xy_1 = 0\]

CBSE: Class 12

Key Points: Second Order Derivative

  • Second derivative means differentiating the function twice with respect to the same variable.

  • It is defined only when the first derivative is differentiable.

  • Common notations are \[\frac{d^2y}{dx^2}\], f''(x), y'', \[D^2y\], and \[y_2\].

  • Higher order derivatives can be defined similarly.

Video Tutorials

We have provided more than 1 series of video tutorials for some topics to help you get a better understanding of the topic.

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