Triangular Geometry and Sine Rule

XAT 2018 · Quantitative Ability & Data Interpretation · Hard · Quantitative Aptitude

A girl travels along a straight line, from point A to B at a constant speed, V1V_1 meters/sec for T seconds. Next, she travels from point B to C along a straight line, at a constant speed of V2V_2 meters/sec for another T seconds. BC makes an angle 105° with AB. If CA makes an angle 30° with BC, how much time will she take to travel back from point C to A at a constant speed of V2V_2 meters/sec, if she travels along a straight line from C to A?

  1. A.

    0.53(3−1)T0.53(\sqrt{3} - 1)T

  2. B.

    TT

  3. C.

    0.5(3+1)T0.5(\sqrt{3} + 1)T

  4. D.

    3\sqrt{3}

  5. E.

    None of the above

Answer

C

Explanation

In △ABC\triangle ABC: AB=V1TAB = V_1 T, BC=V2TBC = V_2 T. Interior angle ∠B=180∘−105∘=75∘\angle B = 180^\circ - 105^\circ = 75^\circ. ∠C=30∘\angle C = 30^\circ. Sum of angles = 180∘  ⟹  ∠A=180∘−75∘−30∘=75∘180^\circ \implies \angle A = 180^\circ - 75^\circ - 30^\circ = 75^\circ. Since ∠A=∠B=75∘\angle A = \angle B = 75^\circ, △ABC\triangle ABC is isosceles with AC=BC=V2TAC = BC = V_2 T or using Sine Rule: ACsin⁡75∘=BCsin⁡75∘  ⟹  AC=BC=V2T\frac{AC}{\sin 75^\circ} = \frac{BC}{\sin 75^\circ} \implies AC = BC = V_2 T. Wait, using altitude BD⊥ACBD \perp AC: BD=V2Tsin⁡30∘=V2T2BD = V_2 T \sin 30^\circ = \frac{V_2 T}{2}. CD=V2Tcos⁡30∘=32V2TCD = V_2 T \cos 30^\circ = \frac{\sqrt{3}}{2} V_2 T. In △ABD\triangle ABD, ∠A=45∘  ⟹  AD=BD=V2T2\angle A = 45^\circ \implies AD = BD = \frac{V_2 T}{2}. Total AC=CD+AD=3+12V2T=0.5(3+1)V2TAC = CD + AD = \frac{\sqrt{3} + 1}{2} V_2 T = 0.5(\sqrt{3} + 1) V_2 T. Time taken at speed V2=ACV2=0.5(3+1)TV_2 = \frac{AC}{V_2} = 0.5(\sqrt{3} + 1)T.

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