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Question
Find the differential equation of the ellipse whose major axis is twice its minor axis.
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Solution
Let 2a and 2b be lengths of major axis and minor axis of the ellipse.
Then 2a = 2(2b)
∴ a = 2b
∴ equation of the ellipse is
`"x"^2/"a"^2 + "y"^2/"b"^2 = 1`
i.e. `"x"^2/(2"b")^2 + "y"^2/"b"^2 = 1`
∴ `"x"^2/(4"b"^2) + "y"^2/"b"^2 = 1`
∴ x2 + 4y2 = 4b2
Differentiating w.r.t. x, we get
`"2x" + 4 xx "2y" "dy"/"dx" = 0`
∴ `"x" + "4y" "dy"/"dx" = 0`
This is the required D.E.
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