Question

Calculați, folosind eventual proprietățile adunarii, sumele S1, S2 :
a) S1= -1+2-3+4-5+6-...-15+16-17+18
b) S2= -1-2+3+4-5-6+7...-14+15+16-17-18

Answer 1

Răspuns:

a) S1= -1+2-3+4-5+6-...-15+16-17+18

S1 = (-1+2)+ (-3+4) + (-5+6) -...(-15+16)+(-17+18)

S1 =   1   + 1 + 1 + ..........+ 1 +1

S1 = 1 * 9 = 9

b)S2= -1-2+3+4-5-6+7...+14-15-16+17+18

S2 = (3+4 - 1-2) + (7+8-5-6) + .....(17 + 18 - 15 - 16)

S2 = (7-3)  + (15 - 11) + .......(35 - 31) - 35

S2 = 4 + 4 + 4 + 4 - 35

S2 = 16 - 35 = -19