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30 +60+ 90 + 120+ ...900

Răspuns :

30 +60+ 90 + 120+ ...900 =

= 30× (1 + 2 + 3 + 4 +...+ 30) =

= 30 × 30 × (30 + 1) : 2 =

= 30 × 15 × 31 =

= 13 950
30+60+90+120+...... +900=

=30 (1+2+...+30)

=30×30×31:2

=13950