Tangent & Normal - Notes, Concept and All Important Formula

TANGENT & NORMAL

1. TANGENT TO THE CURVE AT A POINT:

The tangent to the curve at 'P' is the line through P whose slope is limit of the secant's slope as $Q \rightarrow P$ from either side.

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All Chapter Notes, Concept and Important Formula

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2. NORMAL TO THE CURVE AT A POINT:

A line which is perpendicular to the tangent at the point of contact is called normal to the curve at that point.

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3. THINGS TO REMEMBER :

(a) The value of the derivative at $\mathrm{P}\left(\mathrm{x}_{1}, \mathrm{y}_{1}\right)$ gives the slope of the tangent to the curve at P. Symbolically

$\left.\mathrm{f}^{\prime}\left(\mathrm{x}_{1}\right)=\dfrac{\mathrm{dy}}{\mathrm{dx}}\right]_{\left(\mathrm{x}_{1}, \mathrm{y}_{1}\right)}=$ Slope of tangent at $\mathrm{P}\left(\mathrm{x}_{1}, \mathrm{y}_{1}\right)=\mathrm{m}$ (say).

(b) Equation of tangent at $\left(x_{1}, y_{1}\right)$ is $\left.y-y_{1}=\dfrac{d y}{d x}\right]_{\left(x_{1}, y_{1}\right)}\left(x-x_{1}\right)$

(c) Equation of normal at $\left(x_{1}, y_{1}\right)$ is $\left. y-y_{1}=-\dfrac{1}{\frac{d y}{d x}}\right]_{\left(x_{1}, y_{1}\right)}\left(x-x_{1}\right)$.

Note :

(i) The point $\mathrm{P}\left(\mathrm{x}_{1}, \mathrm{y}_{1}\right)$ will satisfy the equation of the curve $\&$ the equation of tangent $\&$ normal line.

(ii) If the tangent at any point $\mathrm{P}$ on the curve is parallel to the axis of $x$ then $d y / d x=0$ at the point $P$.

(iii) If the tangent at any point on the curve is parallel to the axis of $y$, then $\mathrm{dy} / \mathrm{d} \mathrm{x}$ is not defined or $\mathrm{dx} / \mathrm{dy}=0$ at that point.

(iv) If the tangent at any point on the curve is equally inclined to both the axes then $\mathrm{dy} / \mathrm{dx}=\pm 1$.

(v) If a curve passing through the origin be given by a rational/ integral/algebraic equation, then the equation of the tangent (or tangents) at the origin is obtained by equating to zero the terms of the lowest degree in the equation. e.g. If the equation of a curve be $x^{2}-y^{2}+x^{3}+3 x^{2} y-y^{3}=0$, the tangents at the origin are given by $x^{2}-y^{2}=0$ i.e. $x+y=0$ and $x-y=0$

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4. ANGLE OF INTERSECTION BETWEEN TWO CURVES :

Angle of intersection between two curves is defined as the angle between the two tangents drawn to the two curves at their point of intersection. If the angle between two curves is $90^{\circ}$ then they are called ORTHOGONAL curves.

Note : If the curves $\dfrac{\mathrm{x}^{2}}{\mathrm{a}}+\dfrac{y^{2}}{\mathrm{~b}}=1$ and $\dfrac{\mathrm{x}^{2}}{\mathrm{c}}+\dfrac{\mathrm{y}^{2}}{\mathrm{~d}}=1$, intersect each other orthogonally, then $\mathrm{a}-\mathrm{c}=\mathrm{b}-\mathrm{d}$.

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5. LENGTH OF TANGENT, SUBTANGENT, NORMAL & SUBNORMAL :

(a) Length of the tangent $(\mathrm{PT})=\dfrac{y_{1} \sqrt{1+\left[f^{\prime}\left(x_{1}\right)\right]^{2}}}{f^{\prime}\left(x_{1}\right)}$

(b) Length of Subtangent $(\mathrm{MT})=\dfrac{\mathrm{y}_{1}}{\mathrm{f}^{\prime}\left(\mathrm{x}_{1}\right)}$

(c) Length of Normal (PN) $=y_{1} \sqrt{1+\left[\mathrm{f}^{\prime}\left(\mathrm{x}_{1}\right)\right]^{2}}$

(d) Length of Subnormal (MN) $=y_{1} \mathrm{f}^{\prime}\left(\mathrm{x}_{1}\right)$

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6. DIFFERENTIALS :

The differential of a function is equal to its derivative multiplied by the differential of the independent variable. Thus if, $y=\tan x$ then $d y=\sec ^{2} x d x$. In general $d y=f^{\prime}(x) d x$ or $d f(x)=f^{\prime}(x) d x$

Note :

(i) $\mathrm{d}(\mathrm{c})=0$ where ' $\mathrm{c}$ ' is a constant

(ii) $\mathrm{d}(\mathrm{u}+\mathrm{v})=\mathrm{d} \mathrm{u}+\mathrm{d} \mathrm{v}$

(iii) $\mathrm{d}(\mathrm{uv})=\mathrm{udv}+\mathrm{vdu}$

(iv) $\mathrm{d}(\mathrm{u}-\mathrm{v})=\mathrm{du}-\mathrm{dv}$

(v) $\mathrm{d}\left(\dfrac{\mathrm{u}}{\mathrm{v}}\right)=\dfrac{\mathrm{vdu}-\mathrm{udv}}{\mathrm{v}^{2}}$

(vi) For the independent variable ' $x$ ', increment $\Delta x$ and differential dx are equal but this is not the case with the dependent variable 'y' i.e. $\Delta y \neq d y$. $\therefore$ Approximate value of y when increment $\Delta x$ is given to independent variable $x$ in $y=f(x)$ is $y+\Delta y=f(x+\Delta x)=f(x)+\dfrac{d y}{d x} \cdot \Delta x$

(vii) The relation $\mathrm{d} y=\mathrm{f}^{\prime}(\mathrm{x}) \mathrm{dx}$ can be written as $\dfrac{\mathrm{dy}}{\mathrm{dx}}=\mathrm{f}^{\prime}(\mathrm{x})$; thus the quotient of the differentials of '$y$ ' and 'x' is equal to the derivative of 'y' w.r.t. 'x'.

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