Question:
Two tangent lines l1 and l2 are drawn from the point (2, 0) to the parabola 2y2 = – x. If the lines l1 and l2 are also tangent to the circle (x – 5)2 + y2 = r, then 17r is equal to _________.
Two tangent lines l1 and l2 are drawn from the point (2, 0) to the parabola 2y2 = – x. If the lines l1 and l2 are also tangent to the circle (x – 5)2 + y2 = r, then 17r is equal to _________.
Updated On: Jan 6, 2025
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Solution and Explanation
\(y² = -\frac{x}{2} \)
\(y = mx - \frac{1}{8m} \)
this tangent pass through (2, 0)
\(y = mx - \frac{1}{8m} \)
this tangent pass through (2, 0)
\(\begin{array}{l}\Rightarrow m=\pm \frac{1}{4}\end{array}\)i.e. one tangent is
\(x-4y-2=0 \)
\(17r = 9\)
\(x-4y-2=0 \)
\(17r = 9\)
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Concepts Used:
Parabola
Parabola is defined as the locus of points equidistant from a fixed point (called focus) and a fixed-line (called directrix).
Standard Equation of a Parabola
For horizontal parabola
- Let us consider
- Origin (0,0) as the parabola's vertex A,
- Two equidistant points S(a,0) as focus, and Z(- a,0) as a directrix point,
- P(x,y) as the moving point.
- Let us now draw SZ perpendicular from S to the directrix. Then, SZ will be the axis of the parabola.
- The centre point of SZ i.e. A will now lie on the locus of P, i.e. AS = AZ.
- The x-axis will be along the line AS, and the y-axis will be along the perpendicular to AS at A, as in the figure.
- By definition PM = PS
=> MP2 = PS2
- So, (a + x)2 = (x - a)2 + y2.
- Hence, we can get the equation of horizontal parabola as y2 = 4ax.
For vertical parabola
- Let us consider
- Origin (0,0) as the parabola's vertex A
- Two equidistant points, S(0,b) as focus and Z(0, -b) as a directrix point
- P(x,y) as any moving point
- Let us now draw a perpendicular SZ from S to the directrix.
- Then SZ will be the axis of the parabola. Now, the midpoint of SZ i.e. A, will lie on P’s locus i.e. AS=AZ.
- The y-axis will be along the line AS, and the x-axis will be perpendicular to AS at A, as shown in the figure.
- By definition PM = PS
=> MP2 = PS2
So, (b + y)2 = (y - b)2 + x2
- As a result, the vertical parabola equation is x2= 4by.