A = 1.2 + 2.3 + ... + 99.100
3A = 1.2.3 + 2.3.(4-1) + ... + 99.100.(101-98)
3A = 1.2.3 + 2.3.4 - 1.2.3 + ... + 99.100.101 - 98.99.100
3A = 99.100.101
3A = 999900
A = 333300
C = 1.2.3 + 2.3.4 + ... + 49.50.51
4C = 1.2.3.4 + 2.3.4.(4-1) + ... + 49.50.51.(52-48)
4c = 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + ... + 49.50.51.52 - 48.49.50.51
4C = 49.50.51.52
4C = 6497400
C = 1624350
Ta có :
a=1.2+2.3+3.4+...+99.100
3a=1.2.3+2.3.3+3.4.3+...+99.100.3
3a=1.2.3+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
a=\(\frac{99.100.101}{3}\)
a=333300
Tính c làm tương tự
a = 1.2 + 2.3 + 3.4 + ... + 99.100
3a = 1.2.3 + 2.3.3 + 3.4.3 + ... + 99.100.3
3a = 1.2.3 + 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 . 33 . 100 . 101
a = 33 . 100 . 101
a = 333300
a = 1.2 + 2.3 + 3.4 + ... + 99.100
3a = 1.2.3 + 2.3.3 + 3.4.3 + ... + 99.100.3
3a = 1.2.3 + 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
a = 99.100.101 : 3
a = 333300
c = 1.2.3 + 2.3.4 + 3.4.5 + ... + 49.50.51
4c = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 + ... + 49.50.51.4
4c = 1.2.3.4 + 2.3.4.(5 - 1) + 3.4.5.(6 - 2) + ... + 49.59.51.(52 - 48)
4c = 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + 3.4.5.6 - 2.3.4.5 + ... + 49.50.51.52 - 48.49.50.51
4c = 49.50.51.52 : 4
c = 1624350