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JEE Advanced 2021 · Conic Sections

For any complex number w=c+idw = c + id, let arg⁡(w)∈(−π,π]\arg(w) \in (-\pi, \pi], where i=−1i = \sqrt{-1}. Let α\alpha and β\beta be real numbers such that for all complex numbers z=x+iyz = x + iy satisfying arg⁡(z+αz+β)=π4\arg\left(\frac{z + \alpha}{z + \beta}\right) = \frac{\pi}{4}, the ordered pair (x,y)(x, y) lies on the circle x2+y2+5x−3y+4=0x^{2} + y^{2} + 5x - 3y + 4 = 0. Then which of the following statements is (are) TRUE?

Answers: B. αβ=4\alpha\beta = 4 · D. β=4\beta = 4

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Written solution

x2+y2+5x−3y+4=0x^{2}+y^{2}+5x-3y+4=0

arg⁡ ⁣(z+αz+β)=π4,z=x+iy\arg\!\left(\dfrac{z+\alpha}{z+\beta}\right)=\dfrac{\pi}{4}, \qquad z=x+iy

Find α,β so that this locus = the given circle\text{Find } \alpha, \beta \text{ so that this locus } = \text{ the given circle}

(1) α=−1(2) αβ=4(3) αβ=−4(4) β=4(1)\ \alpha = -1 \quad (2)\ \alpha\beta = 4 \quad (3)\ \alpha\beta = -4 \quad (4)\ \beta = 4

x2+y2+5x−3y+4=0x^{2}+y^{2}+5x-3y+4=0

(x+52)2+(y−32)2=92\left(x+\dfrac{5}{2}\right)^{2}+\left(y-\dfrac{3}{2}\right)^{2}=\dfrac{9}{2}

Centre C ⁣(−52,32),r=32\text{Centre } C\!\left(-\dfrac{5}{2},\dfrac{3}{2}\right),\quad r=\dfrac{3}{\sqrt{2}}

arg⁡(z+α)−arg⁡(z+β)=π4\arg(z+\alpha)-\arg(z+\beta)=\dfrac{\pi}{4}

∠ at z subtended by (−α,−β)=π4\angle \text{ at } z \text{ subtended by } (-\alpha,-\beta) = \dfrac{\pi}{4}

z=−α⇒z+α=0;z=−β⇒z+β=0z=-\alpha \Rightarrow z+\alpha=0;\qquad z=-\beta \Rightarrow z+\beta=0

So the circle passes through the real points −α, −β\text{So the circle passes through the real points } -\alpha,\ -\beta

y=0: x2+5x+4=0⇒(x+1)(x+4)=0y=0:\ x^{2}+5x+4=0 \Rightarrow (x+1)(x+4)=0

x=−1, −4x=-1,\ -4

{−α,−β}={−1,−4}⇒{α,β}={1,4}\{-\alpha,-\beta\}=\{-1,-4\} \Rightarrow \{\alpha,\beta\}=\{1,4\}

αβ=4\alpha\beta = 4

On the arc: arg⁡(z+1)>arg⁡(z+4)\text{On the arc: } \arg(z+1) > \arg(z+4)

⇒α=1,β=4\Rightarrow \alpha = 1,\qquad \beta = 4

(1) α=−1: ×;(2) αβ=4: ✓(1)\ \alpha=-1:\ \times;\qquad (2)\ \alpha\beta=4:\ \checkmark

(3) αβ=−4: ×;(4) β=4: ✓(3)\ \alpha\beta=-4:\ \times;\qquad (4)\ \beta=4:\ \checkmark

Answer: (2) and (4)\boxed{\text{Answer: (2) and (4)}}

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