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Complex Numbers question

2022 · 27 Jun · Shift 2 · Q23
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Complex Numbers question

2022 · 27 Jun · Shift 2 · Q23

JEE MainMathematicsComplex NumbersMCQ+4 / −1
The number of points of intersection of ∣z−(4+3i)∣=2|z - (4 + 3i)| = 2∣z−(4+3i)∣=2 and ∣z∣+∣z−4∣=6|z| + |z - 4| = 6∣z∣+∣z−4∣=6, z ∈\in∈ C, is :
  1. A
    0
  2. B
    1
  3. C
    2
  4. D
    3
View written solutionFree

Correct answer: C

Let z=x+iyz=x+iyz=x+iy.

We interpret the two loci in the Argand plane.

  1. First locus: ∣z−(4+3i)∣=2|z-(4+3i)|=2∣z−(4+3i)∣=2

This is a circle with center (4,3)(4,3)(4,3) and radius 222.

So its equation is

(x−4)2+(y−3)2=4.(x-4)^2+(y-3)^2=4.(x−4)2+(y−3)2=4.
  1. Second locus: ∣z∣+∣z−4∣=6|z|+|z-4|=6∣z∣+∣z−4∣=6

Here,

  • ∣z∣|z|∣z∣ is distance from (0,0)(0,0)(0,0),
  • ∣z−4∣|z-4|∣z−4∣ is distance from (4,0)(4,0)(4,0).

Hence this is an ellipse with foci (0,0)(0,0)(0,0) and (4,0)(4,0)(4,0), and sum of distances equal to 666.

Since for an ellipse, 2a=6  ⟹  a=3,2a=6 \implies a=3,2a=6⟹a=3, and distance between foci is 444, so 2c=4  ⟹  c=2.2c=4 \implies c=2.2c=4⟹c=2. Thus, b2=a2−c2=9−4=5.b^2=a^2-c^2=9-4=5.b2=a2−c2=9−4=5.

The center is (2,0)(2,0)(2,0), so the ellipse is

(x−2)29+y25=1.\frac{(x-2)^2}{9}+\frac{y^2}{5}=1.9(x−2)2​+5y2​=1.
  1. Solve simultaneously

We need intersections of

(x−4)2+(y−3)2=4(1)(x-4)^2+(y-3)^2=4 \tag{1}(x−4)2+(y−3)2=4(1)

and

(x−2)29+y25=1.(2)\frac{(x-2)^2}{9}+\frac{y^2}{5}=1. \tag{2}9(x−2)2​+5y2​=1.(2)

Instead of solving fully algebraically, use geometry first.

The circle center is (4,3)(4,3)(4,3) with radius 222. The ellipse is symmetric about the xxx-axis.

Check whether the circle can meet the lower half of the ellipse:

  • On the circle, lowest point is at y=1y=1y=1.
  • So any intersection must have y≥1y\ge 1y≥1. Thus only upper branch of ellipse matters.

Now parametrize the circle:

x=4+2cos⁡θ,y=3+2sin⁡θ.x=4+2\cos\theta, \qquad y=3+2\sin\theta.x=4+2cosθ,y=3+2sinθ.

Substitute into ellipse:

(2+2cos⁡θ)29+(3+2sin⁡θ)25=1.\frac{(2+2\cos\theta)^2}{9}+\frac{(3+2\sin\theta)^2}{5}=1.9(2+2cosθ)2​+5(3+2sinθ)2​=1.

This is cumbersome, so let us instead compare positions.

  1. Use upper branch of ellipse

From ellipse:

y=5(1−(x−2)29).y=\sqrt{5\left(1-\frac{(x-2)^2}{9}\right)}.y=5(1−9(x−2)2​)​.

From circle (lower branch gives y<3y<3y<3, upper branch gives larger values, but circle itself lies between y=1y=1y=1 and y=5y=5y=5):

y=3±4−(x−4)2.y=3\pm\sqrt{4-(x-4)^2}.y=3±4−(x−4)2​.

Since ellipse upper branch has y≥0y\ge 0y≥0, intersections with the circle occur where

5(1−(x−2)29)=3−4−(x−4)2\sqrt{5\left(1-\frac{(x-2)^2}{9}\right)}=3-\sqrt{4-(x-4)^2}5(1−9(x−2)2​)​=3−4−(x−4)2​

or

5(1−(x−2)29)=3+4−(x−4)2.\sqrt{5\left(1-\frac{(x-2)^2}{9}\right)}=3+\sqrt{4-(x-4)^2}.5(1−9(x−2)2​)​=3+4−(x−4)2​.

Geometrically, the ellipse's top is at (2,5)≈(2,2.236)(2,\sqrt5)\approx(2,2.236)(2,5​)≈(2,2.236), while the circle center is above and right of this. The circle spans x∈[2,6]x\in[2,6]x∈[2,6].

Let us test key points:

  • At x=2x=2x=2, circle gives
(2−4)2+(y−3)2=4  ⟹  4+(y−3)2=4  ⟹  y=3.(2-4)^2+(y-3)^2=4 \implies 4+(y-3)^2=4 \implies y=3.(2−4)2+(y−3)2=4⟹4+(y−3)2=4⟹y=3.

Ellipse gives

09+y25=1  ⟹  y=±5.\frac{0}{9}+\frac{y^2}{5}=1 \implies y=\pm\sqrt5.90​+5y2​=1⟹y=±5​.

No intersection.

  • At x=4x=4x=4, circle gives
y=3±2  ⟹  y=5,1.y=3\pm 2 \implies y=5,1.y=3±2⟹y=5,1.

Ellipse gives

(4−2)29+y25=1  ⟹  49+y25=1  ⟹  y2=259  ⟹  y=±53.\frac{(4-2)^2}{9}+\frac{y^2}{5}=1 \implies \frac49+\frac{y^2}{5}=1 \implies y^2=\frac{25}{9} \implies y=\pm\frac53.9(4−2)2​+5y2​=1⟹94​+5y2​=1⟹y2=925​⟹y=±35​.

So at x=4x=4x=4, ellipse point lies between the two circle points, suggesting crossing.

Now compare functions on interval x∈[2,4]x\in[2,4]x∈[2,4] and [4,5][4,5][4,5].

Define

f(x)=(x−4)2+(5(1−(x−2)29)−3)2−4.f(x)=(x-4)^2+\left(\sqrt{5\left(1-\frac{(x-2)^2}{9}\right)}-3\right)^2-4.f(x)=(x−4)2+(5(1−9(x−2)2​)​−3)2−4.

Intersections correspond to f(x)=0f(x)=0f(x)=0.

Compute signs:

  • At x=2x=2x=2:
f(2)=4+(5−3)2−4=(5−3)2>0.f(2)=4+(\sqrt5-3)^2-4=(\sqrt5-3)^2>0.f(2)=4+(5​−3)2−4=(5​−3)2>0.
  • At x=4x=4x=4:
f(4)=0+(53−3)2−4=(−43)2−4=169−4=−209<0.f(4)=0+\left(\frac53-3\right)^2-4 =\left(-\frac43\right)^2-4 =\frac{16}{9}-4=-\frac{20}{9}<0.f(4)=0+(35​−3)2−4=(−34​)2−4=916​−4=−920​<0.

So there is at least one root in (2,4)(2,4)(2,4).

  • At x=5x=5x=5, ellipse gives
99+y25=1  ⟹  y=0.\frac{9}{9}+\frac{y^2}{5}=1 \implies y=0.99​+5y2​=1⟹y=0.

Then

f(5)=(1)2+(0−3)2−4=1+9−4=6>0.f(5)=(1)^2+(0-3)^2-4=1+9-4=6>0.f(5)=(1)2+(0−3)2−4=1+9−4=6>0.

Since f(4)<0f(4)<0f(4)<0 and f(5)>0f(5)>0f(5)>0, there is at least one root in (4,5)(4,5)(4,5).

Thus there are at least 222 intersections.

Because the upper branch of the ellipse is single-valued and the circle arc over x∈[2,5]x\in[2,5]x∈[2,5] can cross it at most twice here, total intersections are exactly 222.

Therefore, the number of points of intersection is

2.\boxed{2}.2​.

So the correct option is C.

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