Question:

Tangent to the ellipse $\frac{x^2}{32}+\frac{y^2}{18}=1$ having slope $\frac{-3}{4}$ meets the co-ordinate axes in A and B. Find the area of the triangle AOB, where O is the origin

Updated On: Jul 7, 2022
  • 12 units
  • 8 units
  • 24 units
  • 32 units.
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The Correct Option is C

Solution and Explanation

Tangent to the ellipse$ \frac{x^{2}}{a^{2}} +\frac{y^{2}}{b^{2}} = 1 $ with slope $m$ is $y= mx+\sqrt{a^{2}m^{2}+b^{2}} $ Here $a^{2}= 32, b^{2}=18, m= - 3/4 $ $ \therefore$ tangent is $y= -\frac{3}{4}x +\sqrt{32\cdot\frac{9}{16}+18}$ $= -\frac{3}{4}x+6 $ $ \therefore 3x+4y=24$ $\Rightarrow \frac{x}{8}+\frac{y}{6} = 1 $ $\therefore A $ is $\left(8, 0\right) B$ is $\left(0, 6\right)$ $ \therefore$ area of $\Delta AOB = \frac{1}{2} \left(8\right)\left(6\right)$ $= 24$ sq units
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Concepts Used:

Ellipse

Ellipse Shape

An ellipse is a locus of a point that moves in such a way that its distance from a fixed point (focus) to its perpendicular distance from a fixed straight line (directrix) is constant. i.e. eccentricity(e) which is less than unity

Properties 

  • Ellipse has two focal points, also called foci.
  • The fixed distance is called a directrix.
  • The eccentricity of the ellipse lies between 0 to 1. 0≤e<1
  • The total sum of each distance from the locus of an ellipse to the two focal points is constant
  • Ellipse has one major axis and one minor axis and a center

Read More: Conic Section

Eccentricity of the Ellipse

The ratio of distances from the center of the ellipse from either focus to the semi-major axis of the ellipse is defined as the eccentricity of the ellipse.

The eccentricity of ellipse, e = c/a

Where c is the focal length and a is length of the semi-major axis.

Since c ≤ a the eccentricity is always greater than 1 in the case of an ellipse.
Also,
c2 = a2 – b2
Therefore, eccentricity becomes:
e = √(a2 – b2)/a
e = √[(a2 – b2)/a2] e = √[1-(b2/a2)]

Area of an ellipse

The area of an ellipse = πab, where a is the semi major axis and b is the semi minor axis.

Position of point related to Ellipse

Let the point p(x1, y1) and ellipse

(x2 / a2) + (y2 / b2) = 1

If [(x12 / a2)+ (y12 / b2) − 1)]

= 0 {on the curve}

<0{inside the curve}

>0 {outside the curve}