Question

calculati sumele:
a) 1+3+5+...+31
b) 1+11+21+31+...+131​

Answer 1

1 + 3 + .. + 2n - 1 = n²

2n - 1 = 31 => 2n = 32 =. n = 16

1 + 3 + .. + 31 = 16² = 256

1 + 11 + 21 + 31 + .. + 131​ =

aflam numarul de termeni

avem ratia 10 si primul termen 1

n = (131/10 - 1/10) + 1 = 130/10 + 1 = 13 + 1 = 14 termeni

S = (131 + 1)×14/2 = 132×7 = 924