Question 46 of 70intermediate🔧 ApplyLong Answer4 marks

In each of the following Exercises 10 to 20, find the equation for the ellipse that satisfies the given conditions: Centre at (0,0)(0, 0), major axis on the y-axis and passes through the points (3,2)(3, 2) and (1,6)(1, 6).

Correct Answer

The equation of the ellipse is

x210+y240=1\frac{x^2}{10} + \frac{y^2}{40} = 1

or

4x2+y2=40.4x^2 + y^2 = 40.
Exercise: EXERCISE 10.3 | Q: 19 | (Page 20)
For More Understanding

Explanation

This question requires finding the equation of an ellipse with centre at origin and major axis along y-axis. The standard form for such an ellipse is x2b2+y2a2=1\frac{x^2}{b^2} + \frac{y^2}{a^2} = 1, where aa is the semi-major axis.

Since the ellipse passes through two given points, we substitute these coordinates to get two equations in terms of a2a^2 and b2b^2.

Solving these simultaneous equations gives us the values of a2a^2 and b2b^2, which are then substituted back to get the final equation.

Solution Steps

  1. Step 1: Since the centre is at (0,0)(0, 0) and major axis is on the y-axis, the standard equation of the ellipse is

    x2b2+y2a2=1\frac{x^2}{b^2} + \frac{y^2}{a^2} = 1

    where aa is the semi-major axis and bb is the semi-minor axis.

  2. Step 2: Since the ellipse passes through point (3,2)(3, 2), substituting x=3x = 3 and y=2y = 2:

    9b2+4a2=1...(i)\frac{9}{b^2} + \frac{4}{a^2} = 1 \quad \text{...(i)}
  3. Step 3: Since the ellipse passes through point (1,6)(1, 6), substituting x=1x = 1 and y=6y = 6:

    1b2+36a2=1...(ii)\frac{1}{b^2} + \frac{36}{a^2} = 1 \quad \text{...(ii)}
  4. Step 4: Let 1b2=u\frac{1}{b^2} = u and 1a2=v\frac{1}{a^2} = v. Then equations (i) and (ii) become:

    9u+4v=19u + 4v = 1 u+36v=1u + 36v = 1
  5. Step 5: From the second equation, u=1−36vu = 1 - 36v. Substituting in the first equation:

    9(1−36v)+4v=19(1 - 36v) + 4v = 1

    which gives

    9−324v+4v=19 - 324v + 4v = 1

    so

    320v=8320v = 8

    therefore

    v=140v = \frac{1}{40}
  6. Step 6: Then

    u=1−36(140)=1−910=110u = 1 - 36\left(\frac{1}{40}\right) = 1 - \frac{9}{10} = \frac{1}{10}

    Thus a2=40a^2 = 40 and b2=10b^2 = 10.

  7. Step 7: The required equation of the ellipse is

    x210+y240=1\frac{x^2}{10} + \frac{y^2}{40} = 1