x 1=√2a+b,a,b∈Z
x2=√2c+d,c,d∈Z
rule
x 1+x2=√2(a+c)+(b+d),a+c,b+d∈Z
Therefore, x = x1+x2 ∈ a.
Similarly:
x 1x 2 =√2ad+√2bc+2ac+BD =√2(ad+BC)+(2ac+BD),ad+bc∈Z,2ac+bd∈Z
Therefore x = x1x2 ∈ a.
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