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5+10+15+20+..............+100=?


Răspuns :

5+10+15+20+...+100=
=5x1+5x2+5x3+5x4=..+5x20=
=5x(1+2+3+4+...+20)=
=5x20x21:2=
=100x21:2=
=2100:2=1050