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## Homework Statement

## Homework Equations

1/r = a/b

^{2}* (1+e cosθ)

## The Attempt at a Solution

How do you derive 7.27?

I tried to substitute r in the orbit equation by (acosψ - ae) / cosθ and got

(cos ψ - e)/cosθ*(1+e cosθ)=b^2/a^2 I don't know what to do next. Please help, thank you.