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Published on: 13/05/2022
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Take MCQ Maths Test1.
Find the equation of the ellipse in each of the cases given below:
length of latus rectum 4, distance between foci 4 \( \sqrt{ 2}\) , centre (0, 0) and major axis as y - axis.
2.
Find the length of Latus rectum of the parabola y2 = 4ax.
3.
If the equation 3x2+(3−p)xy+qy2−2px = 8pq represents a circle, find p and q. Also determine the centre and radius of the circle.
4.
Find the equation of circles that touch both the axes and pass through (-4, -2) in general form.
5.
Find the centre and radius of the circle 3x2 + (a + 1)y2 + 6x − 9y + a + 4 = 0.
6.
A circle of radius 3 units touches both the axes. Find the equations of all possible circles formed in the general form.
7.
Find the equation of the circle described on the chord 3x + y + 5 = 0 of the circle x2 + y2 = 16 as diameter.
8.
Find the equation of the tangent and normal to the circle x2+y2−6x+6y−8 = 0 at (2, 2) .
9.
The orbit of Halley’s Comet is an ellipse 36.18 astronomical units long and by 9.12 astronomical units wide. Find its eccentricity.
10.
Determine whether the points (-2, 1), (0, 0) and (-4, -3) lie outside, on or inside the circle x2+y2−5x+2y−5 = 0 .
1.
Length of latus rectum = 4,
distance between foci = 4\(\sqrt { 2 } \) major axis is y-axis
Given \(\frac { { 2b }^{ 2 } }{ a } =4\) and distance between foci = 2ae = 4\(\sqrt { 2 } \)
⇒ ae = \(2\sqrt { 2 } \)
⇒a2e2 = 8 ....(1)
\(\frac { { 2b }^{ 2 } }{ a } =4\Rightarrow { b }^{ 2 }=2a\) ...(2)
We know b2 = a2(1 - e2)
b2 = a2 - a2e2
2a = a2 - 8
[using (1) and (2)]
a2 - 2a - 8 = 0
On factorising we get
(a - 4)(a + 2) = 0
a = 4 or-2
a = 4
⇒ [∴ a = -2 is not possible]
⇒ ae = 16
∴ From (2), b2 = 2(4) = 8
Hence, the equation of the ellipse is
\(\frac { { x }^{ 2 } }{ 8 } +\frac { { y }^{ 2 } }{ 16 } =1\) [∵ Major axis is y -axis]
2.
Equation of the parabola is y2 = 4ax
Latus rectum LL′ passes through the focus (a, 0)
Hence the point L is (a, y1)
Therefore y12 = 4a2
Hence y1 = ±2a
The end points of latus rectum are (a, 2a) and (a, -2a)
3.
Given equation of the circle is
3x2 + (3 - p)xy + qy2 - 2px = 8pq
For the circle, co-efficient of xy = 0
⇒ 3-p = 0 ⇒ p = 3
Also, co-efficient of x2 = co-efficient of y2
⇒ 3 = q
∴ Equation of the circle is
3x2 + 3y2 - 6x = 8(3)(3)
3x2 + 3y2 - 6x - 72 = 0
Dividing by 3, we get
x2 + y2 - 2x - 24 = 0
Here 2g = - 2⇒ g = - 1
f = 0 and c = - 24
Centre is (-g, -f) (1, 0)
and r = \(\sqrt { { g }^{ 2 }+{ f }^{ 2 }-c } \)
= \(\sqrt { { (-1 })^{ 2 }+0+24 } \)
= \(\sqrt { 25 } \) = 5 units.
4.
Since the circles touch both the axes. Its equation will be
(x + a)2 + (y + a)2 = a2 ...............(1)
It passes through (-4, -2)
∴ (-4 + a)2 + (-2 + a)2 = a2
\(16+\not a^{2}+8 a+4+a^{2}+4 a=\not a^{2}\)
⇒ a2 + 12a + 20 = 0
⇒ (a + 10)(a + 2) = 0
a = -10 or -2
Case (i):
When a = -10, (1) becomes
(x + 10)2 + (y + 10)2 = 102
\(\Rightarrow x^{2}+\not 100+20 x+y^{2}+\not 100+20 y=160\)
⇒ x2 + y2+ 20x + 20y + 100 = 0
Case (ii):
When a = -2, (1) becomes
⇒ (x + 2)2 + (y + 2)2 = 22
\(x^{2}+4 x+4+y^{2}+4 y+\not 4 = \not 4\)
x2 + y2+ 4x + 4y + 4 = 0
Hence, equation of the circles are
x2 + y2+ 4x + 4y + 4 = 0
or x2 + y2+ 20x + 20y + 100 = 0
5.
Coefficient of x2 = Coefficient of y2 (characteristic (ii) for a second degree equation to represent a circle).
That is, 3 = a + 1 and a = 2 .
Therefore the equation of the circle is
3x2 + 3y2 + 6x − 9y + 6 = 0
x2 + y2 + 2x − 3y + 2 = 0
So, centre is \(\left( -1,\frac { 3 }{ 2 } \right) \) and radius r =\(\sqrt { 1+\frac { 9 }{ 4 } -2 } \)
=\(\sqrt { \frac { 5 }{ 2 } } \)
6.
As the circle touches both the axes, the distance of the centre from both the axes is 3 units, centre can be (±3, ±3) and hence there are four circles with radius 3, and the required equations of the four circles are
x2 + y2 ± 6x ± 6y + 9 = 0.
7.
Equation of the circle passing through the points of intersection of the chord and circle by
Theorem is x2 + y2−16+\(\lambda \)(3x + y + 5) = 0 .
The chord 3x + y + 5 = 0 is a diameter of this circle if the centre\(\left( \frac { -3\lambda }{ 2 } \frac { -\lambda }{ 2 } \right) \) lies on the chord.
So we have 3\(\left( \frac { -3\lambda }{ 2 } \right) \)-\(\frac { -\lambda }{ 2 } \)+5 = 0,
\(\frac { -9\lambda }{ 2 } \)-\(\frac { \lambda }{ 2 } \)+5 = 0,
−5λ + 5 = 0 ,
λ = 1.
Therefore, the equation of the required circle is x2 + y2+3x + y −11 = 0.
8.
Equation of the circle is x2 + y2 − 6x + 6y − 8 = 0
∴ Equation of the tangent at (x1, y1) is
xx1 + yy1 - \(\frac 62\) (x + x1) + \(\frac 62\) (y + y1) - 8 = 0
Given (x1, y1) is (2, 2)
Equation of the tangent at (2, 2) is
x(2) + y(2) - 3(x + 2) + 3(y + 2) - 8 = 0
\(\Rightarrow 2 x-2 y-3 x+\not 6+3 y+\not 6-8=0\)
⇒ -x + 5y - 8 = 0
⇒ x − 5y + 8 = 0
Equation of the normal is
yx1 - xy1 + g(y - y1) - f(x - x1) = 0
⇒ y(2) -x(2) - 3(y - 2) -3(x - 2) = 0
∵ 2g = -6
⇒ g = -3
2f = 6
⇒ f = 3
⇒ 2y - 2x - 3y + 6 - 3x + 6 = 0
⇒ -5x - y + 12 = 0
Another method for Normal:
Equation of tangent is perpendicular to normal
x - 5y + 8 = 0
Perpendicular equation be 5x + y + k = 0
At (2, 2)
10 + 2 + k = 0
k = -12
Therefore 5x + y - 12 = 0 be equation of normal.
9.
Given that 2a = 36.18, 2b = 9.12 , we get
e = \(\sqrt { 1-\frac { { b }^{ 2 } }{ { a }^{ 2 } } } =\frac { \sqrt { { a }^{ 2 }-{ b }^{ 2 } } }{ a } \) = \(\frac { \sqrt { { \left( \frac { 36.18 }{ 2 } \right) }^{ 2 }{ \left( \frac { 9.12 }{ 2 } \right) }^{ 2 } } }{ \frac { 36.18 }{ 2 } } \)
\(\frac { \sqrt { { \left( 18.09 \right) }^{ 2 }-{ \left( 4.56 \right) }^{ 2 } } }{ \left( 8.09 \right) } \approx0.97\)
10.
Given equation of the circle is
x2 + y2 - 5x + 2y - 5 = 0
(i) At (-2, 1), (1) becomes
(-2)2 + 12- 5(-2) + 2(1) - 5
= 4 + 1 + 10 + 2 - 5
= 17 - 5 = 12 > 0
∴ (-2, 1) lies outside the circle.
(ii) At (0, 0), (1) becomes -5 < 0
∴ (0, 0) lies inside the circle.
(iii) At (-4, -3), (1) becomes
(-4)2 + (-3)2 - 5(-4) + 2(-3) - 5
= 16 + 9 + 20 - 6 - 5
= 45 - 11 = 34 > 0
∴ (-4, -3) lies outside the circle.
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