Răspuns :

3+6+9+...+60=

=3(1+2+3+....+20)=

=3×(20×21)/2=

=3×10×21=

=30×21=630

Anonim3618id
3+6+.....+60

Dam factor comun 3!

1•3+2•3+....+20•3
3•(1+2+3+.....+20)
3•(20+19+18+.....+1)
3•(21+21+21+......+21)
3•(21•20:2)
3•(420:2)
3•120=
=360

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