Răspuns:
Explicație pas cu pas:
9 + 10 + 11 + .... + 30 = 429
= ( 1+2+3+4+.... + 8 + 9 + 10 + ...... + 30 ) - ( 1+2+3+4+....+8) =
= 30 × ( 1+30) : 2 - 8 × ( 1+8) : 2 =
= 30 × 31 : 2 - 8 × 9 : 2 =
= 930 : 2 - 72 : 2 =
= 465 - 36 =
= 429
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Sau:
-> aflam cati termeni are suma: 30 - 9 + 1 = 22 termeni
9 + 10 + 11 + ..... + 30 =
= 22 × ( 9 + 30 ) : 2 =
= 11 × 39 =
= 429
