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Equation of Normal to Hyperbola

Equation of Normal to Hyperbola

Edited By Komal Miglani | Updated on Feb 12, 2025 01:05 AM IST

The locus of a point moves in a plane such that the ratio of the distance from a fixed point (focus) to the distance from a fixed line (directrix) is constant. The line perpendicular to the tangent to the curve at the point of contact is normal to the Hyperbola. In real life, we use Hyperbolas in race tracks, architectural design, mirrors, and celestial orbits.

This Story also Contains
  1. What is Hyperbola?
  2. Equation of Normal of Hyperbola in Point form
  3. Equation of Normal of Hyperbola in Slope form
  4. Solved Examples Based on Equation of Normal of Hyperbola
Equation of Normal to Hyperbola
Equation of Normal to Hyperbola

In this article, we will cover the concept of Equation of Normal in Point Form and Parametric Form. This category falls under the broader category of Coordinate Geometry, which is a crucial Chapter in class 11 Mathematics. It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination(JEE Main) and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE, and more. A total of fifteen questions have been asked on JEE MAINS( 2013 to 2023) from this topic including two in 2018, one in 2020, one in 2021, four in 2022, and one in 2023.

Background wave

What is Hyperbola?

A Hyperbola is the set of all points ( x,y ) in a plane such that the difference of their distances from two fixed points is a constant. Each fixed point is called a focus (plural: foci).
OR,
The locus of a point moves in a plane such that the ratio of the distance from a fixed point (focus) to the distance from a fixed line (directrix) is constant. The constant is known as eccentricity e and for hyperbola e 1.

Hyperbola

Equation of Hyperbola

The standard form of the equation of a hyperbola with centre (0,0) and foci lying on the x-axis is x2a2y2b2=1 where b2=a2(e21)

Equation of Normal of Hyperbola in Point form

The equation of normal at (x1,y1) to the hyperbola, x2a2y2b2=1 is a2xx1+b2yy1=a2+b2

Tangent and Normal to Hyperbola

Derivation of Equation of Normal of Hyperbola in Point form

We know that the equation of tangent in point form at (x1,y1)

xx1a2yy1b2=1
Slope of tangent at (x1,y1) is b2x1a2y1
Slope of normal at (x1,y1) is a2y1 b2x1
Hence, the equation of normal at point (x1,y1) is

(yy1)=a2y1 b2x1(xx1)
 or a2xx1+b2yy1=a2+b2
Equation of Normal of Hyperbola in Parametric form

The equation of normal at (asecθ,btanθ) to the hyperbola, x2a2y2b2=1 is axcosθ+bycotθ=a2+b2

Derivation of Equation of Normal of Hyperbola in Parametric form

The equation of normal in point form is a2xx1+b2yy1=a2+b2

 Put (x1,y1)(asecθ,btanθ)a2xasecθ+b2ybtanθ=a2+b2axcosθ+bycotθ=a2+b2
The equation of normal at (asecθ,btanθ) to the hyperbola, x2a2y2b2=1 is axcosθ+bycotθ=a2+b2

The equation of normal in point form is a2xx1+b2yy1=a2+b2 put (x1,y1)(asecθ,btanθ).

a2xasecθ+b2ybtanθ=a2+b2axcosθ+bycotθ=a2+b2

Equation of Normal of Hyperbola in Slope form

The equation of normal of slope m to the hyperbola, x2a2y2b2=1 are y=mxm(a2+b2)a2m2b2 and coordinate of point of contact is (±a2a2m2b2,mb2a2m2b2)

The equation of normal at (asecθ,btanθ) to the hyperbola, x2a2y2b2=1 is axcosθ+bycotθ=a2+b2

Derivation of Equation of Normal of Hyperbola in Slope form

y=asinθbx+a2+b2bcotθ
Let, asinθb=m

Hence, the equation of normal becomes

y=mxm(a2+b2)a2m2b2, where m[ab,ab]
Pair of Tangents

sinθ=bmacotθ=±a2b2m2bm


The combined equation of pair of tangents from the point P(x1,y1) to the hyperbola

x2a2y2b2=1 is (x2a2y2b21)(x12a2y12b21)=(xx1a2yy1b21)2 or, SS1=T2 where, S=x2a2y2 b21 S1=x12a2y12 b221 T=xx1a2yy1 b21
Note:
The formula SS1=T2 can be used to find the combined equation of a pair of tangents for any general hyperbola as well.
Chord of Contact

Chord of contact of Hyperbola

The equation of chord of contact of tangents from the point P(x1,y1) to the hyperbola x2a2y2b2=1 is x1a2y1b2=1
i.e. T=0 which is a chord of contact QR.

Equation of Chord bisected at a given point

Chord of contact of hyperbola bisected at a given point

The equation of chord of the hyperbola κa2b2=1 bisected at a given point P(x1,y1)
is xx1a2yy1 b21=x12a2y2 b21
or, T=S1

Solved Examples Based on Equation of Normal of Hyperbola

Example 1: Let m1, and m2 be the slopes of the tangents drawn from the point P(4,1) to the hyperbola H:y225x216=1. If Q is the point from which the tangents are drawn to H have slopes |m1| and |m2| and they make positive intercepts α and β on the x-axis, then (PQ)2αβ is equal to [JEE MAINS 2023]

Solution: Equation of tangent to the hyperbola y2a2x2 b2=1
y=mx±a2b2 m2
passing through (4,1)
1=4 m±2516 m24 m2m3=0

Equation of tangent with positive slopes 1&34
4y=3x16y=x3} with positive intercept on x-axis.

α=163,β=3
Intersection points:

Q:(4,7)P:(4,1)PQ2:(128)PQ2αβ=12816=8
Hence, the answer is 8.

Example 2: Consider a hyperbola H:x22y2=4. Let the tangent at a point P(4,6) meet the axis at Q and latus rectum at R(x1,y1),x1>0. If F is a focus of H which is nearer to the point P, then the area of ΔQFR is equal to: [JEE MAINS 2021]

Solution

x24y22=1e=1+b2a2=32

Focus F(ae,0)F(6,0)

equation of the tangent at P to the hyperbola is

2xy6=2

tangent meet x-axis at Q(1,0)
latus rectum x=6 at R(6,26(61))
Area of ΔQFR=12(61)26(61)=(61)26=762
Hence, the answer is 762

Example 3: The vertices of a hyperbola H are (±6,0) and its eccentricity is 52. Let N be the normal to H at a point in the first quadrant and parallel to the line 2x+y=22. If d is the length of the line segment of N between H and the y-axis then d2 is equal to [JEE MAINS 2020]
Solution



H:x236y29=1
The equation of normal is 6xcosθ+3ycotθ=45

M=2sinθ=2θ=π/4
The equation of normal is 2x+y=15

P(asecθ,btanθ)P(62,3),k(0,15)d2=216

Hence, the answer is 216


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