If the complex number z1,z2 the origin form an equilateral triangle then z21+z22 =
A
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B
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C
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D
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Solution
The correct option is A Let OA, OB be the sides of an equilateral and let OA, OB represent the complex numbers or vectors z1,z2 respectively.
From the equilateral ΔOAB,¯AB=z2−z1∴arg(z2−z1z1)=arg(z2−z1)−argz2=π3andarg(z2z1)=arg(z2)−arg(z1)=π3Also∣∣z2−z1z2∣∣=1=∣∣z2z1∣∣,sincetriangleisequilateral. Thusthevectorsz2−z1z2andz2z1havesamemodulusandsameargument,whichimpliesthatthevectorsareequal,thatisz2−z1z2=z2z1⇒z1z2−z21=z22⇒z21+z22=z1z2 Note : Students should remember this question as a formula.