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Calculati sumele a) 1+4+7+10+.......+ 139

Răspuns :

1+4+7+10+...+139=1+4+7+..+139=(1+139)*47:2=3290
1+4+7+10+...+139

4=3*1+1
7=3*2+1
10=3*3+1

139=3*46+1

=> 46 termeni

S=46*140:2
S=46*70
S=3220