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Illustration of a parabola - a curve which is the locus of a point that moves in a plane so that its distance from a fixed point (focus) in the plane is always equal to its distance from a fixed line (directrix) in the plane.

Parabola With Focus and Directrix

Illustration of a parabola - a curve which is the locus of a point that moves in a plane so that its…

Illustration of a parabola showing point P is equidistant to the focus and the directrix.

Point on Parabola

Illustration of a parabola showing point P is equidistant to the focus and the directrix.

Illustration of a parabola. "If a line PT is drawn from any point P of the curve, bisecting the angle between PF and the perpendicular from P to the directrix, every point of the line PT, except P, is without the curve."

Point on Parabola

Illustration of a parabola. "If a line PT is drawn from any point P of the curve, bisecting the angle…

Illustration of a tangent line drawn from an external point to a parabola.

Tangent to a Parabola

Illustration of a tangent line drawn from an external point to a parabola.

Illustration showing that the area of a parabolic segment made by a chord is two thirds the area of the triangle formed by the chord and the tangents drawn through the ends of the chord.

Tangents and Chords of a Parabola

Illustration showing that the area of a parabolic segment made by a chord is two thirds the area of…

Illustration showing that if two tangents RP and RQ are drawn from a point R to a parabola, the line drawn through R parallel to the axis bisects the chord of contact.

Tangents to a Parabola

Illustration showing that if two tangents RP and RQ are drawn from a point R to a parabola, the line…

"Hold a lamp reflector or other large concave mirror directly facing the sun, so as to bring the rays of light to a focus...At some point, W, between F and C, the center of curvature of the reflector, hang a loud-ticking watch, and hunt for the point, X, at which the ear can most distinctly hear the ticking. Moving the reflector will render the sound inaudible." -Avery 1895

Reflection of Sound Using a Reflector

"Hold a lamp reflector or other large concave mirror directly facing the sun, so as to bring the rays…