a) Sai

b)Đúng

c) Đúng

D) Sai

a =1.2+3.4+.....+99.100

=>3a=1.2.3+2.3.3+3.4.3+....+99.100.3

=>3a=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...99.100(101-98

=>3a=1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +.....+99.100.101 - 98.99.100

=>3a= 99.100.101

=>3a:3=99.100.101:3

=>a= (99:3).100.101

=>a=33.100.101

=> a=333 300