Răspuns:
Explicație pas cu pas:
7 + 9 + 11 + 13 + ...... + 25 = 160
-> stabilesc cati termeni are suma cu ratia = 2 ( 9-7; 11-9)
(25 - 7 ) : 2 + 1 = 18 : 2 + 1 = 10 termeni are suma
= 10 × ( 7 + 25 ) : 2 =
= 5 × 32 =
= 160
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Sau:
1 + 3 + 5 + 7 + 9 + ....... + 25 - ( 1 + 3 + 5 ) =
( 25 - 1 ) : 2 + 1 = 13 termeni are suma
= 13 × ( 1 + 25 ) : 2 - 9 =
= 13 × 26 : 2 - 9 =
= 13 × 13 - 9 =
= 169 - 9 =
= 160
