Tính nhanh: B = 13 + 14 + 15 + 16 + .... + 98 + 99

1:tính nhanh
a) B=13+14+15+16+........+98+99
b) C=12+14+16+.........+96+98
c) D= 13+15+17+......... +47+49
d) E= 1.2+2.3+3.4+..........+17.18+18.19