Question

Calculează următoarele sume: a)1+2+3+. . .+30; b)2+3+4+. . .+30 ; c)9+10+11+ . . . +30.

Answer 1

1+2+3+..........+n=n(n+1)

1+2+...............30=30(30+1)/2=465

b)

2+3+4+. . .+30=30(30+1)/2-1=464

c)

9+10+11+ . . . +30=30(30+1)/2-8(8+1)/2=465-36=429

Answer 2

a)1+2+3+. . .+30 = (1+30)x30:2 = 31x15 = 465

b)2+3+4+. . .+30 = (2+30)x29:2 = 16x29 = 464

c)9+10+11+ . . . +30 = (9+30)x22:2 = 39x11 = 429