The semi latus rectum formula for conics (2nd order curves) reads: |r|=l/(1+epsilon.r1). r1 is the unit vector in the direction of the r. (epsilon.r1) represents the scalar product of the excentricity vector epsilon with r1. l is the semi-latus rectum = |epsilon| |d|.

Change the directrance d (distance focus - directrix) and the green excentricity vector epsilon interactively. This creates the yellow conic (hyperbola, parabola, ellipse, circle) with radius vector r.

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