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Tangent lines to a parabola. Given a point Ì0(õ0,ó0) on a parabola, it is required to write the equation of a tangent to the parabola at this point.Let us find the slope of the tangent:
To this end, we differentiate equation (20) as an implicit function:
Substituting this into the equation of a straight line with given slope, we obtain
Since the point Ì0 (õ0;ó0) belongs to the parabola, its coordinates satisfy the equation of the parabola:
Thus, we obtain the equation of a tangent to the parabola
Example. Write the classical equation of the parabola with directrix õ=–5 . The parabola is given by the equation Then, the required equation of a parabola is
Definition. The locus of points for which the ratio of the distances to focal radii to the distances to the corresponding directrices is constant and equal to the eccentricity
(1) less than 1, then it is called an ellipse; (2) larger than 1, is called a hyperbola; (3) equal to 1, is called a parabola.
A General Equation of a Second-Order Curve and Its Classical Form
Definition. An equation of the form
is called a general equation of a second-order curve. Assigning particular values to the coefficients, we obtain the above equations of a hyperbola, an ellipse, and a parabola. Date: 2015-01-02; view: 1749
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