arthham

JEE Advanced 2011 · Straight Lines

A straight line LL through the point (3,−2)(3, -2) is inclined at an angle 60∘60^\circ to the line 3x+y=1\sqrt{3}x + y = 1. If LL also intersects the xx-axis, then the equation of LL is

Answer: B. y−3x+2+33=0y - \sqrt{3}x + 2 + 3\sqrt{3} = 0

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Written solution

Given: L passes through (3, −2)\text{Given: } L \text{ passes through } (3,\, -2) L makes 60∘ with 3x+y=1L \text{ makes } 60^\circ \text{ with } \sqrt{3}x + y = 1 L intersects the x-axisL \text{ intersects the } x\text{-axis} To find: the equation of L\text{To find: the equation of } L (from the four given options)\text{(from the four given options)} 1. slope of the given line1.\ \text{slope of the given line} 2. angle formula for L2.\ \text{angle formula for } L 3. x-axis condition, then the equation3.\ x\text{-axis condition, then the equation} 3x+y=1 ⇒ y=−3x+1\sqrt{3}x + y = 1 \ \Rightarrow \ y = -\sqrt{3}x + 1 m1=−3m_1 = -\sqrt{3} tan⁡θ=∣m2−m11+m1m2∣\tan\theta = \left|\dfrac{m_2 - m_1}{1 + m_1 m_2}\right| 3=∣m2+31−3 m2∣\sqrt{3} = \left|\dfrac{m_2 + \sqrt{3}}{1 - \sqrt{3}\,m_2}\right| m2+3=3−3m2m_2 + \sqrt{3} = \sqrt{3} - 3m_2 4m2=0 ⇒ m2=04m_2 = 0 \ \Rightarrow \ m_2 = 0 m2+3=−3+3m2m_2 + \sqrt{3} = -\sqrt{3} + 3m_2 −2m2=−23 ⇒ m2=3-2m_2 = -2\sqrt{3} \ \Rightarrow \ m_2 = \sqrt{3} m2=0orm2=3m_2 = 0 \quad \text{or} \quad m_2 = \sqrt{3} inclination of given line=120∘\text{inclination of given line} = 120^\circ θL=60∘ or 180∘ ⇒ m2=3 or 0\theta_L = 60^\circ \text{ or } 180^\circ \ \Rightarrow \ m_2 = \sqrt{3} \text{ or } 0 m2=0⇒y=−2 (parallel to x-axis)⇒discardm_2 = 0 \Rightarrow y = -2 \ \text{(parallel to } x\text{-axis)} \Rightarrow \text{discard} y−(−2)=3 (x−3)y - (-2) = \sqrt{3}\,(x - 3) y+2=3x−33y + 2 = \sqrt{3}x - 3\sqrt{3} y−3x+2+33=0y - \sqrt{3}x + 2 + 3\sqrt{3} = 0 Answer: (2)  y−3x+2+33=0\boxed{\text{Answer: (2)}\ \ y - \sqrt{3}x + 2 + 3\sqrt{3} = 0}

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