Răspuns:
Explicație pas cu pas:
1 + 5 + 9 + ..... + 41 = 231
-> stabilim cati termeni are suma cu ratia = 4(5-1=4; 9-5=4....)
(41-1):4+1 = 40:4+1 = 11 termeni are suma
= 11 × (1 + 41 ) : 2 =
= 11 × 42 : 2 =
= 11 × 21 =
= 231
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Sau:
1+5+9+13+17+21+25+29+33+37+41 = 231
= (1+41) +(5+37) + (9+33) +(13+29) +(17+25) + 21 =
= 42 + 42 + 42 + 42 + 42 + 21 =
= 5 x 42 + 21 =
= 210 + 21 =
= 231
