Răspuns:
Explicație pas cu pas:
11 + 12 + 13 + ..... + 78 = 3 026
78 - 11 + 1 = 68 termeni are suma
= 68 × ( 11 + 78 ) : 2 =
= ( 68 : 2 ) × 89=
= 34 × 89 =
= 3 026
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Sau:
( 1 + 2 + 3 + ..... + 10 + 11 + 12 + ..... + 78 ) - ( 1 + 2 + 3 + ..... + 10 ) =
= 78 × ( 1 + 78 ) : 2 - 10 × ( 1 + 10 ) : 2 =
= 39 × 79 - 5 × 11 =
= 3 081 - 55 =
= 3 026
