1.

Find the equation of the ellipse, whose length of the major axis is 20 and foci are `(0,+-5)`.

Answer» Since the foci of the ellipse lie on the y-axis, it is a vertical ellipse.
Let the required equation be ` x^(2)/b^(2) + y^(2)/a^(2) = 1," where " a^(2) gt b^(2).`
Let ` c^(2) = (a^(2) -b^(2)).`
Its foci are `(0, pm c) ` and therefore, c = 5.
Also, a = length of the semi-major axis = `(1/2 xx 20) = 10.`
Now, `c^(2) = (a^(2) -b^(2)) hArr b^(2) = (a^(2)-c^(2)) = (100 - 25) = 75`.
Thus, `a^(2) = (10)^(2) = 100 and b^(2) = 75.`
Hence, the required equation is `x^(2)/75 + y^(2)/100 = 1.`


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