CP and CD are conjugate diameters of ellipse `(x^2)/(a^2)+(y^2)/(b^2)=1`. Then the locus of mid-point of PD is (where C is centre of ellipse and P,D are adjacent points)

Options

A. `x^(2)/a^(2) + y^(2)/b^(2) = 2`
B. `x^(2)/a^(2) + y^(2)/b^(2) = 1/2`
C. `x^(2)/a^(2) + y^(2)/b^(2) = 4`
D. `x^(2)/a^(2) + y^(2)/b^(2) = 1/4`

Answer

B

Explanation

Let (h, k) be the mid-point of PD (See Fig. 11)
Then,
`2h = a cos theta - sin theta` and `2k = b sin theta + b cos theta`
`rArr (2h)/(a) = cos theta - sin theta` and `(2k)/(b) = sin theta + cos theta`
`rArr (4h^(2))/(a^(2)) + (4k^(2))/(b^(2)) = (cos theta - sin theta)^(2) + (sin theta + cos theta)^(2)`
`rArr h^(2)/a^(2) + k^(2)/b^(2) = 1/2`
Hence, the locus of (h, k) is `x^(2)/a^(2) + y^(2)/b^(2) = 1/2`.