If the main sequence radius of the sun is 7.0 x10 to the 5th km and the earth orbits the distance of 1.5 x10 to the 8th .How much of the Earth's orbit will the star fill during a red giant phase? is the answer 4.66 x10 to the -3 ?

I suspect it is the ratio of the areas that is wanted, so ratio = (7*10^5/1.5*10^8)^2 → (4.667*10^-3)^2 ≈ 2.178*10^-5

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Thanks! ^.^