Q47 / 108 · NTA Abhyas (JEE Main) 2020 · Mathematics · Complex Number

Let z1=3,z2=2 and z1+z2+z3=3+4i. If the real part of z1z2-+z2z3-+z3z1- is equal to 4, then z3 is equal to (where, i2=-1)

  • A
    1
  • Correct answer
    B
    2
  • C
    3
  • D
    4

Solution

|z1+z2+z3|2=32+42=25
⇒z1+z2+z3z1-+z2-+z3-=25
⇒|z1|2+|z2|2+|z3|2+z1z2-+z2z3-+z3z1-+z1-z2+z2-z3+z3-z1=25
⇒9+4+z32+z1z2-+z2z3-+z3z1-+z1-z2+z2-z3+z3-z1=25
⇒z32+2real(z1z2-+z2z3-+z3z1-)=12
⇒|z3|2=4⇒z3=2

Hence, the correct answer is option: 'B'.

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