Question

Cum se face 7+9+.....+25
Am uitat

Answer 1

Răspuns:

7+9+...+25

r= 9-7=2

s25= 10× (7+25)/2= 10×16= 160.

Answer 2

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

_____________________________________

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