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The orbit of a planet around its star is an​ ellipse, with the star at one focus. The apoapsis of a planet is its greatest distance from the​ star, and the periapsis is its shortest distance. Its mean distance is the length of the semimajor axis. The mean distance from a planet to its star is93 million miles. If the apoapsis of the planet is98 million​ miles, what is the​ periapsis? Write an equation for the orbit of the planet around the star.The periapsis is ____million miles.

Answer :

Solution :

Let the center be (0,0)

Now from the diagram given below ,

The major axis = periapsis + apoapsis

2 x semi major axis = periapsis + 98

2 x 93 = 98 + periapsis

Therefore, periapsis = 186 - 98

                                  = 88 million miles

Star coordinate = (93-98, 0)

                          = (-5,0)

Let the minor axis be 2b

For ellipse [tex]$c^2 = a^2 - b^2$[/tex]

                 [tex]$(-5)^2 = (-93)^2 - b^2$[/tex]

                   [tex]$b^2=8624$[/tex]

                   [tex]$b=\sqrt{8624}$[/tex]

Now, equation of the ellipse

[tex]$\frac{x^2}{(93)^2}+\frac{y^2}{(\sqrt{8624})^2}=1$[/tex]

[tex]$\frac{x^2}{(93)^2}+\frac{y^2}{8624}=1$[/tex]    

The equation for the orbit of the planet around the star is;

(x²/93²) + (y²/(√8624)²) = 1

The length of the periapsis of the given planet is;

88 million miles

We are told the orbit around its' star is an ellipse.

We are told that apoapsis of a planet is its greatest distance from the​ star, and the periapsis is its shortest distance. Thus;

major axis = periapsis + apoapsis

Now, we are told that; the apoapsis of the planet 98 million​ miles.

Also, that The mean distance from a planet to its star is 93 million miles.

Thus, major axis/2 = 93

Thus, major axis = 93 × 2

Major axis = 186 million miles

Thus;

186 = periapsis + 98

periapsis = 186 - 98

periapsis = 88 million miles

To get the equation of the orbit of the planet around the star, lets first find the focus coordinate.

We know it's y-coordinate will be zero since its the focus but it's x-coordinate will be; 93 - 98 = -5

Thus; star coordinate = (-5, 0)

As we are dealing with ellipse, we have gotten;

a = 93 and c = -5

Thus; b = √(a² - c²)

b = √(93² - (-5)²)

b = √8624

Formula for equation of the ellipse is;

(x²/a²) + (y²/b²) = 1

Thus, we have;

(x²/93²) + (y²/(√8624)²) = 1

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