Aus Online Mathematik Brückenkurs 2
In general, the expression 
z−w is equal to the distance between the complex numbers z and w, so if we rewrite the equality in the exercise as
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| z−(1+i)=3
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it says that the distance between z and 1+i should be 3, i.e. we obtain a circle of radius 3 and centre at 1+i.
