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Complex Numbers question

2023 · 13 Apr · Shift 1 · Q39
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  5. /2023 · 13 Apr · Shift 1 · Q39

Complex Numbers question

2023 · 13 Apr · Shift 1 · Q39

JEE MainMathematicsComplex NumbersNumerical+4 / −1
Let w=zzˉ+k1z+k2iz+λ(1+i),k1,k2∈Rw=z \bar{z}+k_{1} z+k_{2} i z+\lambda(1+i), k_{1}, k_{2} \in \mathbb{R}w=zzˉ+k1​z+k2​iz+λ(1+i),k1​,k2​∈R. Let Re⁡(w)=0\operatorname{Re}(w)=0Re(w)=0 be the circle C\mathrm{C}C of radius 1 in the first quadrant touching the line y=1y=1y=1 and the yyy-axis. If the curve Im⁡(w)=0\operatorname{Im}(w)=0Im(w)=0 intersects C\mathrm{C}C at A\mathrm{A}A and B\mathrm{B}B, then 30(AB)230(A B)^{2}30(AB)2 is equal to ‾\underline{\hspace{2cm}}​
Numerical answer
View written solutionFree

Correct answer: 120

Let z=x+iyz=x+iyz=x+iy with x,y∈Rx,y\in\mathbb Rx,y∈R. Then zˉ=x−iy,zzˉ=x2+y2.\bar z=x-iy,\qquad z\bar z=x^2+y^2.zˉ=x−iy,zzˉ=x2+y2.

We are given w=zzˉ+k1z+k2iz+λ(1+i),k1,k2∈R.w=z\bar z+k_1z+k_2iz+\lambda(1+i),\qquad k_1,k_2\in\mathbb R.w=zzˉ+k1​z+k2​iz+λ(1+i),k1​,k2​∈R.

We must use the condition that Re⁡(w)=0\operatorname{Re}(w)=0Re(w)=0 is a circle of radius 111 in the first quadrant touching the line y=1y=1y=1 and the yyy-axis.


1. Write Re⁡(w)\operatorname{Re}(w)Re(w) and Im⁡(w)\operatorname{Im}(w)Im(w)

First compute: k1z=k1x+ik1y,k_1z=k_1x+ik_1y,k1​z=k1​x+ik1​y, iz=i(x+iy)=ix−y,iz=i(x+iy)=ix-y,iz=i(x+iy)=ix−y, so k2iz=−k2y+ik2x.k_2iz=-k_2y+ik_2x.k2​iz=−k2​y+ik2​x. Also, λ(1+i)=λ+iλ.\lambda(1+i)=\lambda+i\lambda.λ(1+i)=λ+iλ. Hence w=(x2+y2)+(k1x−k2y+λ)+i(k1y+k2x+λ).w=(x^2+y^2)+(k_1x-k_2y+\lambda)+i(k_1y+k_2x+\lambda).w=(x2+y2)+(k1​x−k2​y+λ)+i(k1​y+k2​x+λ).

Therefore, Re⁡(w)=x2+y2+k1x−k2y+λ,\operatorname{Re}(w)=x^2+y^2+k_1x-k_2y+\lambda,Re(w)=x2+y2+k1​x−k2​y+λ, Im⁡(w)=k1y+k2x+λ.\operatorname{Im}(w)=k_1y+k_2x+\lambda.Im(w)=k1​y+k2​x+λ.


2. Use the circle condition

The curve Re⁡(w)=0\operatorname{Re}(w)=0Re(w)=0 is x2+y2+k1x−k2y+λ=0.x^2+y^2+k_1x-k_2y+\lambda=0.x2+y2+k1​x−k2​y+λ=0. This is a circle.

For a circle x2+y2+2gx+2fy+c=0,x^2+y^2+2gx+2fy+c=0,x2+y2+2gx+2fy+c=0, center is (−g,−f)(-g,-f)(−g,−f) and radius is g2+f2−c\sqrt{g^2+f^2-c}g2+f2−c​.

Comparing, 2g=k1⇒g=k12,2g=k_1\Rightarrow g=\frac{k_1}{2},2g=k1​⇒g=2k1​​, 2f=−k2⇒f=−k22.2f=-k_2\Rightarrow f=-\frac{k_2}{2}.2f=−k2​⇒f=−2k2​​. So center is (−k12,k22).\left(-\frac{k_1}{2},\frac{k_2}{2}\right).(−2k1​​,2k2​​).

Now the circle has radius 111, lies in the first quadrant, touches the line y=1y=1y=1 and the yyy-axis (x=0x=0x=0).

A circle of radius 111 touching x=0x=0x=0 must have center at distance 111 from the yyy-axis, so xext−coordinateofcenter=1.x ext{-coordinate of center}=1.xext−coordinateofcenter=1. A circle of radius 111 touching y=1y=1y=1 and lying below it in the first quadrant must have center at distance 111 from y=1y=1y=1, hence y-coordinate of center=0.y\text{-coordinate of center}=0.y-coordinate of center=0.

Thus center is (1,0)(1,0)(1,0) and radius is 111. So the circle is (x−1)2+y2=1,(x-1)^2+y^2=1,(x−1)2+y2=1, or x2+y2−2x=0.x^2+y^2-2x=0.x2+y2−2x=0.

Comparing with x2+y2+k1x−k2y+λ=0,x^2+y^2+k_1x-k_2y+\lambda=0,x2+y2+k1​x−k2​y+λ=0, we get k1=−2,k2=0,λ=0.k_1=-2,\qquad k_2=0,\qquad \lambda=0.k1​=−2,k2​=0,λ=0.


3. Equation of Im⁡(w)=0\operatorname{Im}(w)=0Im(w)=0

Now Im⁡(w)=k1y+k2x+λ=0.\operatorname{Im}(w)=k_1y+k_2x+\lambda=0.Im(w)=k1​y+k2​x+λ=0. Substitute the values: −2y=0⇒y=0.-2y=0\Rightarrow y=0.−2y=0⇒y=0.

So the curve Im⁡(w)=0\operatorname{Im}(w)=0Im(w)=0 is the xxx-axis.


4. Intersection of this line with the circle

Circle: (x−1)2+y2=1.(x-1)^2+y^2=1.(x−1)2+y2=1. With y=0y=0y=0, (x−1)2=1(x-1)^2=1(x−1)2=1 so x−1=±1⇒x=0,2.x-1=\pm1\Rightarrow x=0,2.x−1=±1⇒x=0,2.

Hence the intersection points are A=(0,0),B=(2,0).A=(0,0),\qquad B=(2,0).A=(0,0),B=(2,0). So AB=2.AB=2.AB=2.

Therefore, 30(AB)2=30⋅4=120.30(AB)^2=30\cdot 4=120.30(AB)2=30⋅4=120.


5. Compare with stored answer

My derived answer is 120.120.120. The stored correct answer is 242424.

These do not match. The geometry and algebra both consistently give AB=2AB=2AB=2, hence 30(AB)2=12030(AB)^2=12030(AB)2=120.

So I disagree with the stored answer.

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