Question

1+1 x 2+3+2 x 2+5+3 x 2+.......+21+11 x 2+23+12 x 2+25 Calculati sumele

Answer 1

nu iti spune rezulttul?

Answer 2

S = (1·2 + 2·2 + 3·2 +.........+ 12·2) +(1 + 3 + 5 +..........+ 23 + 25)
S1 = 2( 1+2+3 +......+12) = 2·12·13/2 = 156
S2 = 1+3+5+......+25) = 1·13 + 2(1+2+3+........+12) = 13 + 2·12·13/2 = 169
1 = 1
3 = 1 + 1·2
5 = 1 +2·2
...................
25 = 1 + 12·2
S = S1 + S2 = 156 + 169 = 325