Given that, ∣z1+z2∣=∣z1∣+∣z2∣∣z1+z2∣ is the vector sum of z1 and z2. So ∣z1+z2∣ 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