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factor comun 3+6+9+...+90=

Răspuns :

3+6+9+...+90=
=3(1+2+3+...+30)=
=3×31×30:2=1395
3+6+9+...+90=
=3(1+2+...+30)
=3
·30·31:2
=3·15·31
=1395