Maths-
General
Easy

Question

A(z1), B(z2) and C(z3) be the vertices of an equilateral triangle in the argand plane such that ∣z1∣ = ∣z2∣ = ∣z3∣. Then which of the following is false ?

  1. fraction numerator z subscript 2 end subscript plus z subscript 3 end subscript over denominator 2 z subscript 1 end subscript minus z subscript 2 end subscript minus z subscript 3 end subscript end fraction is purely real    
  2. fraction numerator z subscript 2 end subscript minus z subscript 3 end subscript over denominator 2 z subscript 1 end subscript minus z subscript 2 end subscript minus z subscript 3 end subscript end fraction is purely imaginary    
  3. open vertical bar arg invisible function application open parentheses fraction numerator z subscript 1 end subscript over denominator z subscript 2 end subscript end fraction close parentheses close vertical bar equals 2 open vertical bar arg invisible function application open parentheses fraction numerator z subscript 3 end subscript minus z subscript 2 end subscript over denominator z subscript 1 end subscript minus z subscript 2 end subscript end fraction close parentheses close vertical bar    
  4. none of these    

The correct answer is: none of these


    Since ∣ z1 ∣ = ∣ z2 ∣ = ∣ z3
    \OA = OB = OC

    \origin is the circumcentre of equilateral triangle ABC.
    \open vertical bar arg invisible function application open parentheses fraction numerator z subscript 1 end subscript over denominator z subscript 2 end subscript end fraction close parentheses close vertical bar equals angle A O B equals fraction numerator 2 pi over denominator 3 end fraction and open vertical bar arg invisible function application open parentheses fraction numerator z subscript 3 end subscript minus z subscript 2 end subscript over denominator z subscript 1 end subscript minus z subscript 2 end subscript end fraction close parentheses close vertical bar equals angle A B C equals fraction numerator pi over denominator 3 end fraction
    \open vertical bar arg invisible function application open parentheses fraction numerator z subscript 1 end subscript over denominator z subscript 2 end subscript end fraction close parentheses close vertical bar equals 2 open vertical bar arg. invisible function application open parentheses fraction numerator z subscript 3 end subscript minus z subscript 2 end subscript over denominator z subscript 1 end subscript minus z subscript 2 end subscript end fraction close parentheses close vertical bar is true
    Letalpha equals fraction numerator z subscript 2 end subscript plus z subscript 3 end subscript over denominator 2 z subscript 1 end subscript minus z subscript 2 end subscript minus z subscript 3 end subscript end fraction\stack alpha with ̄ on top equals fraction numerator stack z with ̄ on top subscript 2 end subscript plus stack z with ̄ on top subscript 3 end subscript over denominator 2 stack z with ̄ on top subscript 1 end subscript minus stack z with ̄ on top subscript 2 end subscript minus stack z with ̄ on top subscript 3 end subscript end fraction equals fraction numerator fraction numerator r to the power of 2 end exponent over denominator z subscript 2 end subscript end fraction plus fraction numerator r to the power of 2 end exponent over denominator z subscript 3 end subscript end fraction over denominator fraction numerator 2 r to the power of 2 end exponent over denominator z subscript 1 end subscript end fraction minus fraction numerator r to the power of 2 end exponent over denominator z subscript 2 end subscript end fraction minus fraction numerator r to the power of 2 end exponent over denominator z subscript 3 end subscript end fraction end fraction
    equals fraction numerator z subscript 1 end subscript left parenthesis z subscript 2 end subscript plus z subscript 2 end subscript right parenthesis over denominator 2 z subscript 2 end subscript z subscript 3 end subscript minus z subscript 1 end subscript z subscript 3 end subscript minus z subscript 1 end subscript z subscript 2 end subscript end fraction ∣ z subscript 1 end subscript stack z with ̄ on top subscript 1 end subscript plus z subscript 2 end subscript stack z with ̄ on top subscript 2 end subscript equals z subscript 3 end subscript stack z with ̄ on top subscript 3 end subscript blank equals fraction numerator z subscript 2 end subscript plus z subscript 3 end subscript over denominator fraction numerator 2 z subscript 2 end subscript z subscript 3 end subscript over denominator z subscript 1 end subscript end fraction minus z subscript 2 end subscript minus z subscript 3 end subscript end fraction
    Nowstack alpha with ̄ on top equals alpha i f 2 fraction numerator z subscript 2 end subscript z subscript 3 end subscript over denominator z subscript 1 end subscript end fraction equals 2 z subscript 1 end subscript
    i.e. if z subscript 2 end subscript z subscript 3 end subscript equals z subscript 1 end subscript superscript 2 end superscripti.e. ifr e to the power of 2 pi divided by 3 end exponent. r e to the power of 4 pi divided by 3 end exponent equals r to the power of 2 end exponent
    i.e. ifr2 = r2 which is true.
    \alpha is wholly real i..e. (a) is true.
    Similarly (b) is true
    Hence (d) is correct

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