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5.

What is the value of p, for which the ellipse 4x2+py2=804x^2 + py^2 = 80 will pass through the points (0,±4)(0, \pm 4)? Find the coordinates of the foci of the ellipse and the lengths of the axes.

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Value of p:
The equation of the ellipse is:
4x2+py2=804x^2 + py^2 = 80
Since it passes through the points (0,±4)(0, \pm 4):
4(0)2+p(±4)2=804(0)^2 + p(\pm 4)^2 = 80
16p=80  ⟹  p=516p = 80 \implies p = 5

Standard Equation of the Ellipse:
Substituting p=5p = 5 into the equation:
4x2+5y2=804x^2 + 5y^2 = 80
Dividing both sides by 80:
x220+y216=1\frac{x^2}{20} + \frac{y^2}{16} = 1
Here, a2=20  ⟹  a=25a^2 = 20 \implies a = 2\sqrt{5} and b2=16  ⟹  b=4b^2 = 16 \implies b = 4.
Since a>ba > b, the major axis lies along the x-axis.

Eccentricity (e):
e=1−b2a2=1−1620=420=15e = \sqrt{1 - \frac{b^2}{a^2}} = \sqrt{1 - \frac{16}{20}} = \sqrt{\frac{4}{20}} = \frac{1}{\sqrt{5}}

Coordinates of the Foci:
Foci=(±ae,0)=(±25⋅15,0)=(±2,0)\text{Foci} = (\pm ae, 0) = \left(\pm 2\sqrt{5} \cdot \frac{1}{\sqrt{5}}, 0\right) = (\pm 2, 0)

Lengths of the Axes:

  • Length of major axis =2a=2(25)=45= 2a = 2(2\sqrt{5}) = 4\sqrt{5} units
  • Length of minor axis =2b=2(4)=8= 2b = 2(4) = 8 units

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