Răspuns:
aplicam suma lui Gauss
1 + 2 + 3 + 4 + … + n = n·( n + 1 ) : 2
a) 1+2+3+.....+30=[30·(30+1)]/2=30·31:2=465
b) 2+3+4 + … +30=
30-2+1=28+1=29 termenii sumei
[29(2+30)]/2=29·32:2=464
c) 9+10+11 + ...... +30 = 429
30-9+1=21+1=22 termenii sumei
[22(9+30)]/2=22·39:2=429
