For two complex numbers z1 and z2:(az1+b¯¯¯z1)(cz2+d¯¯¯z2)=(cz1+d¯¯¯z1)(az2+b¯¯¯z2) if (a,b,c,dϵR)
A
ab=cd
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B
ad=bc
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C
|z1|=|z2|
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D
arg(z1)=arg(z2)
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Solution
The correct options are Aab=cd Darg(z1)=arg(z2) (az1+b¯¯¯z1)(cz2+d¯¯¯z2)=(cz1+d¯¯¯z1)(az2+b¯¯¯z2) ⇒acz1z2+adz1¯¯¯¯¯z2+bc¯¯¯¯¯z1z2+bd¯¯¯¯¯z1¯¯¯¯¯z2=acz1z2+bcz1¯¯¯¯¯z2+ad¯¯¯¯¯z1z2+bd¯¯¯¯¯z1¯¯¯¯¯z2 ⇒(bc−ad)(z2¯¯¯¯¯z1−z1¯¯¯¯¯z2)=0 ⇒ either bc−ad=0 or z2¯¯¯¯¯z1−z1¯¯¯¯¯z2=0 ⇒bc=ad or z1¯¯¯¯¯z1=z2¯¯¯¯¯z2 i.e. ab=cd or arg(z1)=arg(z2)