Section 8

Tangents to conic sections

[33/79%]
Show
LaTeX
Problem 8.1

Find the equation of the tangent line to the curve:

?
(1)

x212+y24=1\frac{x^{2}}{12} + \frac{y^{2}}{4} = 1 at the point (3,1)(3, 1)

(2)

x236+y212=1\frac{x^{2}}{36} + \frac{y^{2}}{12} = 1 at the point (3,−3)(3, - 3)

(3)

x29−y212=1\frac{x^{2}}{9} - \frac{y^{2}}{12} = 1 at the point (−3,0)(- 3, 0)

(4)

x232−y28=1\frac{x^{2}}{32} - \frac{y^{2}}{8} = 1 at the point (6,1)(6, 1)

(5)

xy=8xy = 8 at the point (4,2)(4, 2)

(6)

y2=6xy^{2} = 6x at the point (3/2,3)(3 / 2, 3).

Problem 8.2

Find the equation of the tangent line to the curve:

?
(1)

(x−α)2a2+(y−β)2b2=1\frac{(x - \alpha )^{2}}{a^{2}} + \frac{(y - \beta )^{2}}{b^{2}} = 1

(2)

(x−α)2a2−(y−β)2b2=1\frac{(x - \alpha )^{2}}{a^{2}} - \frac{(y - \beta )^{2}}{b^{2}} = 1

(3)

xy=kxy = k

(4)

(y−β)2=2p(x−α)(y - \beta )^{2} = 2p(x - \alpha ) at the point (x0,y0)\left(x_{0}, y_{0}\right) belonging to the given curve.

Problem 8.3

Under what necessary and sufficient condition is the line Ax+By+C=0Ax + By + C = 0 tangent to:

?
(1)

the ellipse x2a2+y2b2=1\frac{x^{2}}{a^{2}} + \frac{y^{2}}{b^{2}} = 1;

(2)

the hyperbola x2a2−y2b2=1\frac{x^{2}}{a^{2}} - \frac{y^{2}}{b^{2}} = 1

(3)

the hyperbola x2a2−y2b2=−1\frac{x^{2}}{a^{2}} - \frac{y^{2}}{b^{2}} = - 1;

(4)

the hyperbola xy=kxy = k

(5)

the parabola y2=2px?y^{2} = 2px?

Problem 8.4

Under what necessary and sufficient condition is the vector l(α,β)\mathbf{l}(\alpha , \beta ) a direction vector of some tangent line to the hyperbola x2a2−y2b2=1\frac{x^{2}}{a^{2}} - \frac{y^{2}}{b^{2}} = 1?

?
Problem 8.5

Verify that the given line is tangent to the given curve, and find the coordinates of the point of tangency:

?
(1)

3x−2y−24=0,x248+y236=13x - 2y - 24 = 0, \frac{x^{2}}{48} + \frac{y^{2}}{36} = 1

(2)

3x−y−12=0,x220−y236=13x - y - 12 = 0, \frac{x^{2}}{20} - \frac{y^{2}}{36} = 1

(3)

3x−16y+24=0,xy=−33x - 16y + 24 = 0, \quad xy = - 3

(4)

x+y+1=0,y2=4xx + y + 1 = 0, y^{2} = 4x.

Problem 8.6

Find the equations of the tangent lines to the ellipse x230+y224=1\frac{x^{2}}{30} + \frac{y^{2}}{24} = 1

?
(1)

parallel to the line 2x−y−1=02x - y - 1 = 0

(2)

perpendicular to this same line

(3)

making an angle of 45∘45^{\circ } with the line x+3y+3=0x + 3y + 3 = 0.

Problem 8.7

Find the equations of the tangent lines to the hyperbola x225−y216=1\frac{x^{2}}{25} - \frac{y^{2}}{16} = 1, parallel to the line:

?
(1)

4x=3y4x = 3y

(2)

x=1x = 1

(3)

x−2y+1=0x - 2y + 1 = 0.

Problem 8.8

Find the equation of the tangent line to the parabola y2=10xy^{2} = 10x, perpendicular to the line:

?
(1)

2x+y−4=02x + y - 4 = 0

(2)

y=3y = 3

(3)

x=0x = 0.

Problem 8.9

Which points on the given curve of the second order are at the smallest distance from the given line? Find this distance.

?
(1)

2728x2+97y2=1,3x+4y+5=0\frac{27}{28} x^{2} + \frac{9}{7} y^{2} = 1, 3x + 4y + 5 = 0

(2)

2728x2+97y2=1,3x+4y=0\frac{27}{28} x^{2} + \frac{9}{7} y^{2} = 1, 3x + 4y = 0

(3)

6x2−5y2=19,12x+5y−6=06x^{2} - 5y^{2} = 19, 12x + 5y - 6 = 0

(4)

6x2−5y2=19,12x+5y=06x^{2} - 5y^{2} = 19, 12x + 5y = 0

(5)

y2=64x,4x−3y−76=0y^{2} = 64x, 4x - 3y - 76 = 0.

Problem 8.10

Given the ellipse x2+2y2=1x^{2} + 2y^{2} = 1. Find the distances:

?
(1)

from the foci of the ellipse to the tangent line to it at the point A(1/3,2/3)A(1 / 3, 2 / 3)

(2)

between the tangent lines to the ellipse parallel to the line x+y=1x + y = 1.

Problem 8.11

Find the equation of an ellipse whose axes coincide with the coordinate axes, if it:

?
(1)

contains the point A(−3,2)A(- 3, 2) and is tangent to the line 4x−6y−25=04x - 6y - 25 = 0

(2)

is tangent to the lines x+y−5=0x + y - 5 = 0 and x+4y−10=0x + 4y - 10 = 0.

Problem 8.12

Find the equation of a hyperbola whose axes coincide with the coordinate axes, if it:

?
(1)

contains the point A(4,−22)A(4, - 2\sqrt{2}) and is tangent to the line 3x+y+8=03x + y + 8 = 0;

(2)

is tangent to the lines x=1x = 1 and 5x−2y+3=05x - 2y + 3 = 0.

Problem 8.13

Find the equation of a hyperbola with asymptotes 3x±y=0\sqrt{3} x\pm y = 0, tangent to the line 2x−y−3=02x - y - 3 = 0.

?
Problem 8.14

Find the equation of a parabola:

?
(1)

symmetric with respect to the axis OyOy and tangent to the lines y+2x=0,8x−2y−3=0y + 2x = 0, \quad 8x - 2y - 3 = 0

(2)

given by a canonical equation and tangent to the line x+y+1=0x + y + 1 = 0.

Problem 8.15

Find the equations of the normals to the ellipse x24+y22=1\frac{x^{2}}{4} + \frac{y^{2}}{2} = 1 making an angle of 45∘45^{\circ } with its major axis.

?
Problem 8.16

Find the equation of the tangent line to the parabola y2=−8xy^{2} = - 8x whose segment between the point of tangency and the directrix is bisected by the axis Oy.

?
Problem 8.17

Let OO be the vertex of a parabola, MM an arbitrary point of it, l1l_{1} and l2l_{2} the tangent lines to the parabola at the points OO and MM, NN the point of intersection of the lines l1l_{1} and l2,Pl_{2}, P the projection of the segment OMOM onto l1l_{1}. Prove that the point NN bisects the segment OPOP. Indicate the resulting method for constructing the tangent line to a parabola.

?
Problem 8.18

Prove that:

?
(1)

the segment of a tangent line to a hyperbola between its asymptotes is bisected by the point of tangency

(2)

all triangles formed by the asymptotes of a hyperbola and an arbitrary tangent line to it have one and the same area; express this area in terms of the semi-axes of the hyperbola.

Problem 8.19

Prove that the chord joining the points of tangency of an ellipse (a hyperbola) with two parallel lines passes through the center of the curve.

?
Problem 8.20

Prove that the midpoints of the chords of an ellipse (a hyperbola) parallel to a given line l1l_{1} lie on one line l2l_{2}. Moreover, the tangent lines to the curve at the points where it meets the line l2l_{2} are parallel to the line l1l_{1}.

?
Problem 8.21

Find the equations of the sides of a square circumscribed about the ellipse x25+y24=1\frac{x^{2}}{5} + \frac{y^{2}}{4} = 1.

?
Problem 8.22

Find the equations of the sides of an equilateral triangle circumscribed about the ellipse x29+y2=1\frac{x^{2}}{9} + y^{2} = 1, if:

?
(1)

one of the vertices of the triangle lies on the axis OxOx

(2)

one of the vertices of the triangle lies on the axis OyOy.

Problem 8.23

Under what necessary and sufficient condition can two tangent lines be drawn through the point M0(x0,y0)M_{0}\left(x_{0}, y_{0}\right):

?
(1)

to the ellipse x2a2+y2b2=1\frac{x^{2}}{a^{2}} + \frac{y^{2}}{b^{2}} = 1

(2)

to the hyperbola x2a2−y2b2=1\frac{x^{2}}{a^{2}} - \frac{y^{2}}{b^{2}} = 1

(3)

to the parabola y2=2px?y^{2} = 2px?

Problem 8.24

Find the equations of the tangent lines to the ellipse x218+y28=1\frac{x^{2}}{18} + \frac{y^{2}}{8} = 1 passing through the point:

?
(1)

(−6,0)(- 6, 0)

(2)

(2.7,7)(2.7, \sqrt{7})

(3)

(−4,−232)\left(- 4, - \frac{2}{3} \sqrt{2}\right)

(4)

(1,2)(1, 2)

Problem 8.25

Find the equations of the tangent lines to the hyperbola x24−y2=1\frac{x^{2}}{4} - y^{2} = 1 passing through the point:

?
(1)

(−2,2)(- 2, 2)

(2)

(1.6,0)(1.6, 0)

(3)

(4,3)(4, \sqrt{3})

(4)

(4,1)(4, 1)

(5)

(8,4)(8, 4)

(6)

(0,0)(0, 0)

Problem 8.26

Find the equations of the tangent lines to the parabola y2=16xy^{2} = 16x passing through the point:

?
(1)

(1,−2)(1, - 2)

(2)

(1,4)(1, 4)

(3)

(1,5)(1, 5)

If there are two such tangent lines, compute the area of the triangle formed by the tangent lines and the directrix.

Problem 8.27

Through the point M0(x0,y0)M_{0}\left(x_{0}, y_{0}\right) two tangent lines are drawn to a curve of the second order. Prove that the line passing through the points of tangency is given by the equation:

?
(1)

xx0a2+yy0b2=1\frac{xx_{0}}{a^{2}} + \frac{yy_{0}}{b^{2}} = 1 for the ellipse x2a2+y2b2=1\frac{x^{2}}{a^{2}} + \frac{y^{2}}{b^{2}} = 1

(2)

xx0a2−yy0b2=1\frac{xx_{0}}{a^{2}} - \frac{yy_{0}}{b^{2}} = 1 for the hyperbola x2a2−y2b2=1\frac{x^{2}}{a^{2}} - \frac{y^{2}}{b^{2}} = 1

(3)

yy0=p(x+x0)yy_{0} = p\left(x + x_{0}\right) for the parabola y2=2pxy^{2} = 2px.

Problem 8.28

Find the equations of the common tangent lines to two curves of the second order:

?
(1)

x220+y25=1\frac{x^{2}}{20} + \frac{y^{2}}{5} = 1 and x280+4y25=1\frac{x^{2}}{80} + \frac{4y^{2}}{5} = 1

(2)

x25−y24=1\frac{x^{2}}{5} - \frac{y^{2}}{4} = 1 and x24−y23=1\frac{x^{2}}{4} - \frac{y^{2}}{3} = 1

(3)

x23+y2=1\frac{x^{2}}{3} + y^{2} = 1 and y2=2xy^{2} = 2x

(4)

x23−y2=1\frac{x^{2}}{3} - y^{2} = 1 and y2=2xy^{2} = 2x

(5)

y2=2xy^{2} = 2x and x23−y22=−1\frac{x^{2}}{3} - \frac{y^{2}}{2} = - 1

(6)

x26+y23=1\frac{x^{2}}{6} + \frac{y^{2}}{3} = 1 and x225−y216=1\frac{x^{2}}{25} - \frac{y^{2}}{16} = 1

(7)

y2=89xy^{2} = \frac{8}{9} x and y2=x−1y^{2} = x - 1.

Problem 8.29

Prove that:

?
(1)

the normal to an ellipse at an arbitrary point of it bisects the angle formed by the rays issuing from this point and passing through the foci of the ellipse

(2)

the tangent line to a hyperbola at an arbitrary point of it bisects the angle formed by the rays issuing from this point and passing through the foci of the hyperbola

(3)

the normal to a parabola at an arbitrary point of it bisects the angle formed by the ray issuing from this point and passing through the focus of the parabola, and the ray issuing from this point, lying inside the parabola and parallel to its axis.

Problem 8.30

Prove that:

?
(1)

the tangent lines at the points of intersection of an ellipse and a hyperbola having common foci are mutually perpendicular

(2)

the tangent lines at the points of intersection of two parabolas with a common focus and oppositely directed axes are mutually perpendicular.

Problem 8.31

From an arbitrary point of the directrix of a curve of the second order two tangent lines are drawn to this curve. Prove that the line joining the points of tangency passes through the focus corresponding to this directrix.

?
Problem 8.32

Find the equations of the tangent lines to the curve 6xy+8y2−12x−26y+11=06xy + 8y^{2} - 12x - 26y + 11 = 0:

?
(1)

parallel to the line 6x+17y−4=06x + 17y - 4 = 0

(2)

perpendicular to the line 41x−24y+3=041x - 24y + 3 = 0

(3)

parallel to the line y=2y = 2.

Problem 8.33

Find the equations of the tangent lines to the curve 5x2+6xy+5y2−16x−16y−16=05x^{2} + 6xy + 5y^{2} - 16x - 16y - 16 = 0 passing through the point:

?
(1)

(3,3)(3, 3)

(2)

(0,−0.8)(0, -0.8)

(3)

(0,1)(0, 1).