Given that, |z1+z2|=|z1|+|z2| |z1+z2| is the vector sum of z1 and z2. So |z1+z2
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| should be <|z1|+|z2| but here they are equal so z1 and z2 are collinear. S if z1 makes an angle θ with x axis then z2 will also make θ angle. ∴arg‌z1−arg‌z2=θ−θ=0