Page 1
Solved Examples on Parabola
Q1: The equation of parabola whose focus is (?? , ?? ) and directrix is ?? ?? -
?? ?? + ?? = ?? , is
(a) (?? ?? + ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(b) (?? ?? - ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(c) (?? ?? + ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(d) (?? ?? - ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
Ans: (a)
Sol:
?? ?? 2
= ?? ?? 2
? (?? - 5)
2
+ (?? - 3)
2
= (
3?? - 4?? + 1
v9 + 16
)
2
? 25(?? 2
+ 25 - 10?? + ?? 2
+ 9 - 6?? )
= 9?? 2
+ 16?? 2
+ 1 - 12???? + 6?? - 8?? - 12????
? 16?? 2
+ 9?? 2
- 256?? - 142 ?? + 24???? + 849 = 0
? (4?? + 3?? )
2
- 256?? - 142 ?? + 849 = 0
Q2: The point on the parabola ?? ?? = ?? ?? . Whose distance from the focus is 8 ,
has ?? -coordinate as
(a) 0
(b) 2
(c) 4
(d) 6
Ans: (d)
Sol: If ?? (?? 1
, ?? 1
) is a point on the parabola ?? 2
= 4???? and S is its focus, then ???? =
Page 2
Solved Examples on Parabola
Q1: The equation of parabola whose focus is (?? , ?? ) and directrix is ?? ?? -
?? ?? + ?? = ?? , is
(a) (?? ?? + ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(b) (?? ?? - ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(c) (?? ?? + ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(d) (?? ?? - ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
Ans: (a)
Sol:
?? ?? 2
= ?? ?? 2
? (?? - 5)
2
+ (?? - 3)
2
= (
3?? - 4?? + 1
v9 + 16
)
2
? 25(?? 2
+ 25 - 10?? + ?? 2
+ 9 - 6?? )
= 9?? 2
+ 16?? 2
+ 1 - 12???? + 6?? - 8?? - 12????
? 16?? 2
+ 9?? 2
- 256?? - 142 ?? + 24???? + 849 = 0
? (4?? + 3?? )
2
- 256?? - 142 ?? + 849 = 0
Q2: The point on the parabola ?? ?? = ?? ?? . Whose distance from the focus is 8 ,
has ?? -coordinate as
(a) 0
(b) 2
(c) 4
(d) 6
Ans: (d)
Sol: If ?? (?? 1
, ?? 1
) is a point on the parabola ?? 2
= 4???? and S is its focus, then ???? =
?? 1
+ ??
Here 4?? = 8 ? ?? = 2; ???? = 8
? 8 = ?? 1
+ 2 ? ?? 1
= 6
Q3: If the parabola ?? ?? = ?? ax passes through (-?? , ?? ), then length of its latus
rectum is
(a) ?? /??
(b) ?? /??
(c) ?? /??
(d) 4
Ans: (c)
Sol: The point (-3,2) will satisfy the equation ?? 2
= 4???? ? 4 = -12?? ? Latus
rectum = 4|?? | = 4 × |-
1
3
| =
4
3
Q4: The equation of the parabola with its vertex at the origin, axis on the ?? -
axis and passing through the point (?? , -?? ) is
(a) ?? ?? = ???? ?? + ??
(b) ?? ?? = ???? ??
(c) ?? ?? = -???? ??
(d) ?? ?? = -???? ?? + ??
Ans: (c)
Sol: Since the axis of parabola is ?? -axis with its vertex at origin.
? Equation of parabola ?? 2
= 4???? . Since it passes through (6, -3) ;
? 36 = -12?? ? ?? = -3
? Equation of parabola is ?? 2
= -12?? .
Q5: The line ?? - ?? = ?? is the directrix of the parabola ?? ?? - ???? + ?? = ?? . Then
one of the values of ?? is
(a)
?? ??
(b) 8
(c) 4
Page 3
Solved Examples on Parabola
Q1: The equation of parabola whose focus is (?? , ?? ) and directrix is ?? ?? -
?? ?? + ?? = ?? , is
(a) (?? ?? + ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(b) (?? ?? - ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(c) (?? ?? + ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(d) (?? ?? - ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
Ans: (a)
Sol:
?? ?? 2
= ?? ?? 2
? (?? - 5)
2
+ (?? - 3)
2
= (
3?? - 4?? + 1
v9 + 16
)
2
? 25(?? 2
+ 25 - 10?? + ?? 2
+ 9 - 6?? )
= 9?? 2
+ 16?? 2
+ 1 - 12???? + 6?? - 8?? - 12????
? 16?? 2
+ 9?? 2
- 256?? - 142 ?? + 24???? + 849 = 0
? (4?? + 3?? )
2
- 256?? - 142 ?? + 849 = 0
Q2: The point on the parabola ?? ?? = ?? ?? . Whose distance from the focus is 8 ,
has ?? -coordinate as
(a) 0
(b) 2
(c) 4
(d) 6
Ans: (d)
Sol: If ?? (?? 1
, ?? 1
) is a point on the parabola ?? 2
= 4???? and S is its focus, then ???? =
?? 1
+ ??
Here 4?? = 8 ? ?? = 2; ???? = 8
? 8 = ?? 1
+ 2 ? ?? 1
= 6
Q3: If the parabola ?? ?? = ?? ax passes through (-?? , ?? ), then length of its latus
rectum is
(a) ?? /??
(b) ?? /??
(c) ?? /??
(d) 4
Ans: (c)
Sol: The point (-3,2) will satisfy the equation ?? 2
= 4???? ? 4 = -12?? ? Latus
rectum = 4|?? | = 4 × |-
1
3
| =
4
3
Q4: The equation of the parabola with its vertex at the origin, axis on the ?? -
axis and passing through the point (?? , -?? ) is
(a) ?? ?? = ???? ?? + ??
(b) ?? ?? = ???? ??
(c) ?? ?? = -???? ??
(d) ?? ?? = -???? ?? + ??
Ans: (c)
Sol: Since the axis of parabola is ?? -axis with its vertex at origin.
? Equation of parabola ?? 2
= 4???? . Since it passes through (6, -3) ;
? 36 = -12?? ? ?? = -3
? Equation of parabola is ?? 2
= -12?? .
Q5: The line ?? - ?? = ?? is the directrix of the parabola ?? ?? - ???? + ?? = ?? . Then
one of the values of ?? is
(a)
?? ??
(b) 8
(c) 4
(d)
?? ??
Ans: (c)
Sol: The parabola is ?? 2
= 4
?? 4
(?? -
8
?? ). Putting ?? = ?? , ?? -
8
?? = ?? . The equation is
?? 2
= 4 ·
?? 4
· ??
? The directrix is ?? +
?? 4
= 0 i.e., ?? -
8
?? +
?? 4
= 0. But ?? - 1 = 0 is the directrix. So
8
?? -
?? 4
= 1 ? ?? = -8,4
Q6: The ends of a line segment are ?? (?? , ?? ) and ?? (?? , ?? ). ?? is a point on the
line segment ???? such that ???? : ???? = ?? : ?? . If ?? is an interior point of the
parabola ?? ?? = ?? ?? , then
(a) ?? ? (?? , ?? )
(b) ?? ? (-
?? ?? , ?? )
(c) ?? ? (
?? ?? ,
?? ?? )
(d) None of these
Ans: (a)
Sol: ?? = (1,
1+3?? 1+?? ) It is an interior point of ?? 2
- 4?? = 0 iff (
1+3?? 1+?? )
2
- 4 < 0
? (
1 + 3?? 1 + ?? - 2) (
1 + 3?? 1 + ?? + 2) < 0 ? (
?? - 1
1 + ?? ) (
5?? + 3
1 + ?? ) < 0 ? (?? - 1)(?? +
3
5
) < 0
Therefore, -
3
5
< ?? < 1. But ?? > 0 ? 0 < ?? < 1 ? ?? ? (0,1).
Q7: The equation of the tangent to the parabola ?? ?? = ???? ?? , which is
perpendicular to the line ?? = ?? ?? + ?? is
(a) ?? - ?? ?? + ?? = ??
(b) ?? ?? - ?? + ???? = ??
(c) ?? ?? + ?? - ???? = ??
(d) ?? ?? + ?? + ???? = ??
Ans: (a)
Page 4
Solved Examples on Parabola
Q1: The equation of parabola whose focus is (?? , ?? ) and directrix is ?? ?? -
?? ?? + ?? = ?? , is
(a) (?? ?? + ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(b) (?? ?? - ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(c) (?? ?? + ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(d) (?? ?? - ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
Ans: (a)
Sol:
?? ?? 2
= ?? ?? 2
? (?? - 5)
2
+ (?? - 3)
2
= (
3?? - 4?? + 1
v9 + 16
)
2
? 25(?? 2
+ 25 - 10?? + ?? 2
+ 9 - 6?? )
= 9?? 2
+ 16?? 2
+ 1 - 12???? + 6?? - 8?? - 12????
? 16?? 2
+ 9?? 2
- 256?? - 142 ?? + 24???? + 849 = 0
? (4?? + 3?? )
2
- 256?? - 142 ?? + 849 = 0
Q2: The point on the parabola ?? ?? = ?? ?? . Whose distance from the focus is 8 ,
has ?? -coordinate as
(a) 0
(b) 2
(c) 4
(d) 6
Ans: (d)
Sol: If ?? (?? 1
, ?? 1
) is a point on the parabola ?? 2
= 4???? and S is its focus, then ???? =
?? 1
+ ??
Here 4?? = 8 ? ?? = 2; ???? = 8
? 8 = ?? 1
+ 2 ? ?? 1
= 6
Q3: If the parabola ?? ?? = ?? ax passes through (-?? , ?? ), then length of its latus
rectum is
(a) ?? /??
(b) ?? /??
(c) ?? /??
(d) 4
Ans: (c)
Sol: The point (-3,2) will satisfy the equation ?? 2
= 4???? ? 4 = -12?? ? Latus
rectum = 4|?? | = 4 × |-
1
3
| =
4
3
Q4: The equation of the parabola with its vertex at the origin, axis on the ?? -
axis and passing through the point (?? , -?? ) is
(a) ?? ?? = ???? ?? + ??
(b) ?? ?? = ???? ??
(c) ?? ?? = -???? ??
(d) ?? ?? = -???? ?? + ??
Ans: (c)
Sol: Since the axis of parabola is ?? -axis with its vertex at origin.
? Equation of parabola ?? 2
= 4???? . Since it passes through (6, -3) ;
? 36 = -12?? ? ?? = -3
? Equation of parabola is ?? 2
= -12?? .
Q5: The line ?? - ?? = ?? is the directrix of the parabola ?? ?? - ???? + ?? = ?? . Then
one of the values of ?? is
(a)
?? ??
(b) 8
(c) 4
(d)
?? ??
Ans: (c)
Sol: The parabola is ?? 2
= 4
?? 4
(?? -
8
?? ). Putting ?? = ?? , ?? -
8
?? = ?? . The equation is
?? 2
= 4 ·
?? 4
· ??
? The directrix is ?? +
?? 4
= 0 i.e., ?? -
8
?? +
?? 4
= 0. But ?? - 1 = 0 is the directrix. So
8
?? -
?? 4
= 1 ? ?? = -8,4
Q6: The ends of a line segment are ?? (?? , ?? ) and ?? (?? , ?? ). ?? is a point on the
line segment ???? such that ???? : ???? = ?? : ?? . If ?? is an interior point of the
parabola ?? ?? = ?? ?? , then
(a) ?? ? (?? , ?? )
(b) ?? ? (-
?? ?? , ?? )
(c) ?? ? (
?? ?? ,
?? ?? )
(d) None of these
Ans: (a)
Sol: ?? = (1,
1+3?? 1+?? ) It is an interior point of ?? 2
- 4?? = 0 iff (
1+3?? 1+?? )
2
- 4 < 0
? (
1 + 3?? 1 + ?? - 2) (
1 + 3?? 1 + ?? + 2) < 0 ? (
?? - 1
1 + ?? ) (
5?? + 3
1 + ?? ) < 0 ? (?? - 1)(?? +
3
5
) < 0
Therefore, -
3
5
< ?? < 1. But ?? > 0 ? 0 < ?? < 1 ? ?? ? (0,1).
Q7: The equation of the tangent to the parabola ?? ?? = ???? ?? , which is
perpendicular to the line ?? = ?? ?? + ?? is
(a) ?? - ?? ?? + ?? = ??
(b) ?? ?? - ?? + ???? = ??
(c) ?? ?? + ?? - ???? = ??
(d) ?? ?? + ?? + ???? = ??
Ans: (a)
Sol: A line perpendicular to the given line is 3?? + ?? = ?? ? ?? = -
1
3
?? +
?? 3
Here ?? = -
1
3
, ?? =
?? 3
. If we compare ?? 2
= 16?? with ?? 2
= 4???? , then ?? = 4
Condition for tangency is ?? =
?? ?? ?
?? 3
=
4
(-1/3)
? ?? = -36 … Required equation is
?? + 3?? + 36 = 0.
Q8: Two tangents are drawn from the point (-?? , -?? ) to the parabola ?? ?? =
?? ?? . If ?? is the angle between these tangents, then tan ?? =
(a) 3
(b) ?? /??
(c) 2
(d) ½
Ans: (a)
Sol: Equation of pair of tangent from (-2, -1) to the parabola is given by SS
1
=
?? 2
i.e. (?? 2
- 4?? )(1 + 8) = [?? (-1) - 2(?? - 2)]
2
? 9?? 2
- 36?? = [-?? - 2?? + 4]
2
? 9?? 2
- 36?? = ?? 2
+ 4?? 2
+ 16 + 4???? - 16?? - 8?? ? 4?? 2
- 8?? 2
+ 4???? + 20?? - 8?? + 16 = 0
? tan ?? = |
2vh
2
- ????
?? + ?? | = |
2v4 - 4(-8)
4 - 8
| = |
12
-4
| = 3
Q9: If (
?? ?? )
?? /?? + (
?? ?? )
?? /?? =
v?? ?? , then the angle of intersection of the parabola ?? ?? =
?? ???? and ?? ?? = ?? ???? at a point other than the origin is
(a) ?? /??
(b) ?? /??
(c) ?? /??
(d) None of these
Ans: (b)
Sol:
Given parabolas are ?? 2
= 4???? … ..(i) and ?? 2
= 4???? …… (ii)
These meet at the points (0,0), (4?? 1/3
?? 2/3
, 4?? 2/3
?? 1/3
)
Tangent to (i) at (4?? 1/3
?? 2/3
, 4?? 2/3
?? 1/3
) is ?? . 4?? 2/3
?? 1/3
= 2?? (?? + 4?? 2/3
?? 1/3
)
Page 5
Solved Examples on Parabola
Q1: The equation of parabola whose focus is (?? , ?? ) and directrix is ?? ?? -
?? ?? + ?? = ?? , is
(a) (?? ?? + ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(b) (?? ?? - ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(c) (?? ?? + ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
(d) (?? ?? - ?? ?? )
?? - ?????? ?? - ?????? ?? + ?????? = ??
Ans: (a)
Sol:
?? ?? 2
= ?? ?? 2
? (?? - 5)
2
+ (?? - 3)
2
= (
3?? - 4?? + 1
v9 + 16
)
2
? 25(?? 2
+ 25 - 10?? + ?? 2
+ 9 - 6?? )
= 9?? 2
+ 16?? 2
+ 1 - 12???? + 6?? - 8?? - 12????
? 16?? 2
+ 9?? 2
- 256?? - 142 ?? + 24???? + 849 = 0
? (4?? + 3?? )
2
- 256?? - 142 ?? + 849 = 0
Q2: The point on the parabola ?? ?? = ?? ?? . Whose distance from the focus is 8 ,
has ?? -coordinate as
(a) 0
(b) 2
(c) 4
(d) 6
Ans: (d)
Sol: If ?? (?? 1
, ?? 1
) is a point on the parabola ?? 2
= 4???? and S is its focus, then ???? =
?? 1
+ ??
Here 4?? = 8 ? ?? = 2; ???? = 8
? 8 = ?? 1
+ 2 ? ?? 1
= 6
Q3: If the parabola ?? ?? = ?? ax passes through (-?? , ?? ), then length of its latus
rectum is
(a) ?? /??
(b) ?? /??
(c) ?? /??
(d) 4
Ans: (c)
Sol: The point (-3,2) will satisfy the equation ?? 2
= 4???? ? 4 = -12?? ? Latus
rectum = 4|?? | = 4 × |-
1
3
| =
4
3
Q4: The equation of the parabola with its vertex at the origin, axis on the ?? -
axis and passing through the point (?? , -?? ) is
(a) ?? ?? = ???? ?? + ??
(b) ?? ?? = ???? ??
(c) ?? ?? = -???? ??
(d) ?? ?? = -???? ?? + ??
Ans: (c)
Sol: Since the axis of parabola is ?? -axis with its vertex at origin.
? Equation of parabola ?? 2
= 4???? . Since it passes through (6, -3) ;
? 36 = -12?? ? ?? = -3
? Equation of parabola is ?? 2
= -12?? .
Q5: The line ?? - ?? = ?? is the directrix of the parabola ?? ?? - ???? + ?? = ?? . Then
one of the values of ?? is
(a)
?? ??
(b) 8
(c) 4
(d)
?? ??
Ans: (c)
Sol: The parabola is ?? 2
= 4
?? 4
(?? -
8
?? ). Putting ?? = ?? , ?? -
8
?? = ?? . The equation is
?? 2
= 4 ·
?? 4
· ??
? The directrix is ?? +
?? 4
= 0 i.e., ?? -
8
?? +
?? 4
= 0. But ?? - 1 = 0 is the directrix. So
8
?? -
?? 4
= 1 ? ?? = -8,4
Q6: The ends of a line segment are ?? (?? , ?? ) and ?? (?? , ?? ). ?? is a point on the
line segment ???? such that ???? : ???? = ?? : ?? . If ?? is an interior point of the
parabola ?? ?? = ?? ?? , then
(a) ?? ? (?? , ?? )
(b) ?? ? (-
?? ?? , ?? )
(c) ?? ? (
?? ?? ,
?? ?? )
(d) None of these
Ans: (a)
Sol: ?? = (1,
1+3?? 1+?? ) It is an interior point of ?? 2
- 4?? = 0 iff (
1+3?? 1+?? )
2
- 4 < 0
? (
1 + 3?? 1 + ?? - 2) (
1 + 3?? 1 + ?? + 2) < 0 ? (
?? - 1
1 + ?? ) (
5?? + 3
1 + ?? ) < 0 ? (?? - 1)(?? +
3
5
) < 0
Therefore, -
3
5
< ?? < 1. But ?? > 0 ? 0 < ?? < 1 ? ?? ? (0,1).
Q7: The equation of the tangent to the parabola ?? ?? = ???? ?? , which is
perpendicular to the line ?? = ?? ?? + ?? is
(a) ?? - ?? ?? + ?? = ??
(b) ?? ?? - ?? + ???? = ??
(c) ?? ?? + ?? - ???? = ??
(d) ?? ?? + ?? + ???? = ??
Ans: (a)
Sol: A line perpendicular to the given line is 3?? + ?? = ?? ? ?? = -
1
3
?? +
?? 3
Here ?? = -
1
3
, ?? =
?? 3
. If we compare ?? 2
= 16?? with ?? 2
= 4???? , then ?? = 4
Condition for tangency is ?? =
?? ?? ?
?? 3
=
4
(-1/3)
? ?? = -36 … Required equation is
?? + 3?? + 36 = 0.
Q8: Two tangents are drawn from the point (-?? , -?? ) to the parabola ?? ?? =
?? ?? . If ?? is the angle between these tangents, then tan ?? =
(a) 3
(b) ?? /??
(c) 2
(d) ½
Ans: (a)
Sol: Equation of pair of tangent from (-2, -1) to the parabola is given by SS
1
=
?? 2
i.e. (?? 2
- 4?? )(1 + 8) = [?? (-1) - 2(?? - 2)]
2
? 9?? 2
- 36?? = [-?? - 2?? + 4]
2
? 9?? 2
- 36?? = ?? 2
+ 4?? 2
+ 16 + 4???? - 16?? - 8?? ? 4?? 2
- 8?? 2
+ 4???? + 20?? - 8?? + 16 = 0
? tan ?? = |
2vh
2
- ????
?? + ?? | = |
2v4 - 4(-8)
4 - 8
| = |
12
-4
| = 3
Q9: If (
?? ?? )
?? /?? + (
?? ?? )
?? /?? =
v?? ?? , then the angle of intersection of the parabola ?? ?? =
?? ???? and ?? ?? = ?? ???? at a point other than the origin is
(a) ?? /??
(b) ?? /??
(c) ?? /??
(d) None of these
Ans: (b)
Sol:
Given parabolas are ?? 2
= 4???? … ..(i) and ?? 2
= 4???? …… (ii)
These meet at the points (0,0), (4?? 1/3
?? 2/3
, 4?? 2/3
?? 1/3
)
Tangent to (i) at (4?? 1/3
?? 2/3
, 4?? 2/3
?? 1/3
) is ?? . 4?? 2/3
?? 1/3
= 2?? (?? + 4?? 2/3
?? 1/3
)
Slope of the tangent (?? 1
) =
2?? 4?? 2/3
?? 1/3
=
?? 1/3
2?? 1/3
Tangent to (ii) at (4?? 1/3
?? 2/3
, 4?? 2/3
?? 1/3
) is ?? · 4?? 1/3
?? 2/3
= 2?? (?? + 4?? 2/3
?? 1/3
)
Slope of the tangent (?? 2
) =
2?? 1/3
?? 1/3
If ?? is the angle between the two tangents, then ? tan ?? = |
?? 1
-?? 2
1+?? 1
?? 2
| =
|
?? 1/3
2?? 1/3
-
2?? 1/3
?? 1/3
1+
?? 1/3
2?? 1/3
·
2?? 1/3
?? 1/3
|
=
3
2
·
1
(
?? ?? )
1/3
+ (
?? ?? )
1/3
=
3
2
·
1
v3
2
= v3; ? ?? = 60
°
=
?? 3
Q10: The equation of the common tangent touching the circle (?? - ?? )
?? +
?? ?? = ?? and the parabola ?? ?? = ?? ?? above the ?? -axis, is
(a) v?? ?? = ?? ?? + ??
(b) v?? ?? = -(?? + ?? )
(c) v?? ?? = ?? + ??
(d) v?? ?? = -(?? ?? + ?? )
Ans: (c)
Sol:
Any tangent to ?? 2
= 4?? is ?? = ???? +
1
?? . It touches the circle if 3 = |
3?? +
1
?? v1+?? 2
| or
9(1 + ?? 2
) = (3?? +
1
?? )
2
or
1
?? 2
= 3, ? ?? = ±
1
v3
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