- I correct a little typo in my last answer.

With your notation:

OP:PH = k:1-k

you mean:

OP = k OH

so P = G for k=1:3 (and not for k=-2!)

Friendly

François >You are right.

Yes, it's an error of mine.

>I don't see the link between your

>question and the answer of M.Rousee

>except both are about affine geometry.

I have forgotten to change the subject.

Etienne

[Non-text portions of this message have been removed]- Dear friends

Beeing given 3 lines L_{1}, L_{2}, L_{3} in R^3 with equations:

x = a_{k} z + b_{k}

y = c_{k} z + d_{k}

for k= 1, 2, 3

1° Is there a simple way to write down an equation of the ruled

quadric having these 3 lines as generators?

2° Is it possible to write easily equations for the two systems

of generators of this quadric?

Friendly

François