Change in speed around elliptical orbit

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To find the speed of a comet at a distance of 6e12 m from the Sun, two methods can be used: balancing centrifugal and gravitational forces or applying conservation of energy principles. The centrifugal force, which depends on speed, must equal the gravitational force, which varies with distance. Alternatively, the total mechanical energy, comprising kinetic and potential energy, remains constant throughout the orbit. Clarification is needed regarding the definition of radius in an elliptical orbit, as it refers to the distance from the Sun to specific points on the ellipse. Understanding these concepts is crucial for calculating the comet's speed at the given distance.
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A comet is in an elliptical orbit around the Sun. Its closest approach to the Sun is a distance of 5e10 m (inside the orbit of Mercury), at which point its speed is 9e4 m/s. Its farthest distance from the Sun is far beyond the orbit of Pluto. What is its speed when it is 6e12 m from the Sun? (This is the approximate distance of Pluto from the Sun.)

how do I find the speed? (equations needed)
 
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At any point in the orbit the centrifugal force (depends on speed) must balance the gravitational force (depends on radius)

The energy is constant during the orbit, kinetic energy (depends on speed) + potential energy (depends on radius) must be the same
 
i have a question, because its an ellipse how do you determine the radius, or do you mean the distance from the sun to the two points on the ellipse?
 

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