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JEE Advanced 2011 · Complex Numbers and Quadratic Equations

Let (x0,y0)(x_0, y_0) be the solution of the following equations (2x)ln⁡2=(3y)ln⁡3;3ln⁡x=2ln⁡y(2x)^{\ln 2} = (3y)^{\ln 3} ; 3^{\ln x} = 2^{\ln y}. Then x0x_0 is

Answer: C. 12\frac{1}{2}

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Written solution

Let (x₀, y₀) be the solution of the following equations (2x)^(ln 2) = (3y)^(ln 3) ; 3^(ln x) = 2^(ln y). Then x₀ is (2x)ln⁡2=(3y)ln⁡3(2x)^{\ln 2} = (3y)^{\ln 3} 3ln⁡x=2ln⁡y3^{\ln x} = 2^{\ln y} To find x0\text{To find } x_0 (1) 16(2) 13(3) 12(4) 6(1)\ \frac{1}{6} \qquad (2)\ \frac{1}{3} \qquad (3)\ \frac{1}{2} \qquad (4)\ 6 1. take ln⁡ on both equations1.\ \text{take } \ln \text{ on both equations} 2. set p=ln⁡2, q=ln⁡3, u=ln⁡x, v=ln⁡y2.\ \text{set } p=\ln 2,\ q=\ln 3,\ u=\ln x,\ v=\ln y 3. eliminate v, solve for u, then x=eu3.\ \text{eliminate } v,\ \text{solve for } u,\ \text{then } x=e^u (ln⁡x)(ln⁡3)=(ln⁡y)(ln⁡2)(\ln x)(\ln 3) = (\ln y)(\ln 2) p=ln⁡2,q=ln⁡3,u=ln⁡x,v=ln⁡yp = \ln 2, \quad q = \ln 3, \quad u = \ln x, \quad v = \ln y qu=pv⋯(A)q u = p v \qquad \cdots (A) (ln⁡2)(ln⁡2+ln⁡x)=(ln⁡3)(ln⁡3+ln⁡y)(\ln 2)(\ln 2 + \ln x) = (\ln 3)(\ln 3 + \ln y) p(p+u)=q(q+v)⋯(B)p(p+u) = q(q+v) \qquad \cdots (B) v=qp uv = \frac{q}{p}\,u p(p+u)=q(q+qpu)p(p+u) = q\left(q + \frac{q}{p}u\right) p2+pu=q2+q2pup^2 + p u = q^2 + \frac{q^2}{p}u u(p−q2p)=q2−p2u\left(p - \frac{q^2}{p}\right) = q^2 - p^2 p2−q2p u=−(p2−q2)  ⟹  u=−p\frac{p^2-q^2}{p}\,u = -(p^2-q^2) \implies u = -p u=ln⁡x=−p=−ln⁡2=ln⁡12u = \ln x = -p = -\ln 2 = \ln\frac{1}{2} x=eu=12x = e^{u} = \frac{1}{2} v=qp(−p)=−q  ⟹  y=13v = \frac{q}{p}(-p) = -q \implies y = \frac{1}{3} (2x)ln⁡2=1ln⁡2=1=1ln⁡3=(3y)ln⁡3  ✓(2x)^{\ln 2} = 1^{\ln 2} = 1 = 1^{\ln 3} = (3y)^{\ln 3}\ \ \checkmark ∴ x0=12⇒option (3)\therefore\ x_0 = \frac{1}{2} \Rightarrow \boxed{\text{option (3)}}

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