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The question title says it all.

I am looking for a geometric proof for the fact that the group law defined on elliptic curves is associative.

I've heard somewhere about something on the internet about 2 cubics intersecting at 8 points must have a ninth point in common but I've never understood what that meant. Maybe this is what I'm looking for.

If a well-known reference has this result and I could easily find it in a library (or better yet over the internet, which I did not manage to find), then I'd accept this as an answer too.

Thanks in advance,

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    Take a look to page 6 of http://web.math.ku.dk/~kiming/lecture_notes/2000-2001-elliptic_curves/grouplaw.pdf2018-12-11
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    @JeanMarie : Thanks!2018-12-12

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