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If a point P is exterior to a circle with center O, and if the tangent lines from P touch the circle at points T and S, then ∠TPS and ∠TOS are supplementary (sum to 180°). If a chord TM is drawn from the tangency point T of exterior point P and ∠ PTM ≤ 90° then ∠ PTM = ½ ∠ TOM .
The area of a regular polygon is half its perimeter multiplied by the distance from its center to its sides, and because the sequence tends to a circle, the corresponding formula–that the area is half the circumference times the radius–namely, A = 1 / 2 × 2πr × r, holds for a circle.
If a tangent from an external point A meets the circle at F and a secant from the external point A meets the circle at C and D respectively, then AF 2 = AC × AD (tangent–secant theorem). The angle between a chord and the tangent at one of its endpoints is equal to one half the angle subtended at the centre of the circle, on the opposite side ...
When they are tangent, they form a quadruple of tangent circles with the -axis and with the circle for their mediant (+) / (+). [ 35 ] The Ford circles belong to a special Apollonian gasket with root quadruple ( 0 , 0 , 1 , 1 ) {\displaystyle (0,0,1,1)} , bounded between two parallel lines, which may be taken as the x {\displaystyle x} -axis ...
A tangent, a chord, and a secant to a circle. The intuitive notion that a tangent line "touches" a curve can be made more explicit by considering the sequence of straight lines (secant lines) passing through two points, A and B, those that lie on the function curve. The tangent at A is the limit when point B approximates or tends to A. The ...
The ex-tangential quadrilateral is closely related to the tangential quadrilateral (where the four sides are tangent to a circle). Another name for an excircle is an escribed circle, [3] but that name has also been used for a circle tangent to one side of a convex quadrilateral and the extensions of the adjacent two sides. In that context all ...
The formula for the area of a trapezoid can be simplified using Pitot's theorem to get a formula for the area of a tangential trapezoid. If the bases have lengths a, b, and any one of the other two sides has length c, then the area K is given by the formula [2] (This formula can be used only in cases where the bases are parallel.)
A circle with 1st-order contact (tangent) A circle with 2nd-order contact (osculating) A circle with 3rd-order contact at a vertex of a curve. For each point S(t) on a smooth plane curve S, there is exactly one osculating circle, whose radius is the reciprocal of κ(t), the curvature of S at t.