It is an easy exercise that using ruler and compass one find the focus of a given parabola.
Can one do the same using only a ruler? -- if not, why?
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Sign up to join this communityIt is an easy exercise that using ruler and compass one find the focus of a given parabola.
Can one do the same using only a ruler? -- if not, why?
No, because there exists a projective map which preserves the parabola but does not preserve its focus (this is so for any conic and any point.)