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Let R=Z(i) be the ring of gaussian integers and let A=(2+i)R denote the ideal of all multiples of 2+i Describe the cosets of R/A

im just having trouble understaning this step:

"Since 2+i is in A we have i+A=-2+A"

and then it does it again "Since 5 is in A 5+A=0+A"

why is this?

thanks