Let y = f(x)
First order derivative
$\large \frac{dy}{dx} = f'(x) = y_1 $
Second order derivative
$\large \frac{d^2y}{dx^2} = f”(x) = y_2 $
Third order derivative
$\large \frac{d^3y}{dx^3} = f ”'(x) = y_3 $
nth order derivative
$\large \frac{d^ny}{dx^n} = f^{n}(x) = y_n $
Illustration :
(i) If y3 − 3ax2 + x3 = 0 , show that
$\large \frac{d^2y}{dx^2} + \frac{2a^2 x^2}{y^5} = 0$
(ii) If y = (sin−1x)2 + k sin−1x , show that
$\large (1-x^2)\frac{d^2y}{dx^2} – x\frac{dy}{dx} = 2$
Solution: (i) y3 − 3ax2 + x3 = 0
$\large 3y^2 \frac{dy}{dx} – 6ax + 3 x^2 = 0$
$\large y^2 \frac{dy}{dx} – 2ax + x^2 = 0$
$\large y^2 \frac{dy}{dx} = 2ax – x^2 = 0$
$\large \frac{dy}{dx} = \frac{2ax – x^2}{y^2} $ …(ii)
Again diff. w. r. to x , we get
$\large \frac{d^2y}{dx^2} = \frac{y^2(2a – 2x)-(2ax-x^2)2y\frac{dy}{dx}}{y^4} $
$\large \frac{d^2y}{dx^2} = \frac{y^2(2a – 2x)-(2ax-x^2)2y(\frac{2ax – x^2}{y^2})}{y^4} $
$\large \frac{d^2y}{dx^2} = \frac{y^3(2a-2x)-2(2ax-x^2)^2}{y^5}$
$\large \frac{d^2y}{dx^2} = -\frac{2 a^2 x^2}{y^5}$
$\large \frac{d^2y}{dx^2} + \frac{2 a^2 x^2}{y^5} = 0$
(ii) If y = (sin−1x)2 + k sin−1x
$\large \frac{dy}{dx} = 2 sin^{-1}x . \frac{1}{\sqrt{1-x^2}} + k \frac{1}{\sqrt{1-x^2}}$
$\large \sqrt{1-x^2}\frac{dy}{dx} = 2 sin^{-1}x + k $
Again, differentiating both sides w.r.t. x , we get
$\large \sqrt{1-x^2}\frac{d^2y}{dx^2} + \frac{1}{2\sqrt{1-x^2}}(-2x) \frac{dy}{dx} = \frac{2}{\sqrt{1-x^2}} + 0$
$\large (1-x^2)\frac{d^2y}{dx^2} – x \frac{dy}{dx} = 2 $
Exercise :
(i) Find the derivative w. r. to x of the following functions
(a) $\large y = log(sin\sqrt{x^2 + 1})$
(b) $\large y = \frac{logx}{1+xlogx} $
(c) y = xx2
(d) y = cos−1√cosx
(e) $\large y = log(\frac{1+\sqrt{x}}{1-\sqrt{x}}) $
(f) x cos y = sin (x + y)
(g) $\large y = \frac{5 tan^2 x – 1}{tanx} $
(ii) If y = x logy , prove that
$\large \frac{dy}{dx} = \frac{y^2}{x(y-x)}$
(iii) If $\large x = \frac{3at}{1+t^2}$ and , $\large y = \frac{3at^2}{1+t^3}$
Show that $\large \frac{dy}{dx} = \frac{(1+t^2)^2}{(1+t^3)^2} . \frac{t(2-t^3)}{(1-2t^2)}$
Also Read
| →Limits of a Function →Continuity of a Function →Differentiability of a Function →Rules for differentiation →Higher order derivatives →L’HOSPITAL’S RULE |