4A=4.[1.2.3 + 2.3.4 + 3.4.5 + 4.5.6 + . . . . . . + 48.49.50] 
4A=1.2.3.4 +2.3.4.4 +3.4.5.4 +4.5.6.4 +.........+48.49.50.4 
4A=1.2.3.4 +2.3.4.(5-1) + 3.4.5.(6-2) +4.5.6(7-3)+....+ 48.49.50(51-47) 
4A=1.2.3.4 +2.3.4.5 --1.2.3.4 + 3.4.5.6--2.3.4.5 + 4.5.6.7-3.4.5.6+....+ 48.49.50.51--47.48.49.50 
4A =48.49.50.51 
A=(48.49.50.51)/4 

đáp số bài này là 1.499.400

Đặt A = 1.2+2.3 +3.4 +...+1999.2000

=> 3A = 1.2.(3 - 0) + 2.3.(4 - 1) + ..... + 1999.2000.(2001 - 1998)

=> 3A = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ..... + 1999.2000.2001

=> 3A = 1999.2000.2001

=> A = 1999.2000.2001 / 3

=> A = 2 666 666 000

ko biết nhưng k mk nha

\(A=1.2+2.3+3,4+...+1999.2000\)

\(=>3A=1.2.3+2.3.3+3.4.3+...+1999.2000.3\)

\(=1.2.\left(3-0\right)+2.3.\left(4-1\right)+3.4.\left(5-2\right)+.....+1999.2000.\left(2001-1998\right)\)

\(=1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+...+1999.2000.2001-1998.1999.2000\)

\(=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+1999.2000.2001-1998.1999.2000\)

\(=1999.2000.2001\)

\(=>A=\frac{1999.2000.2001}{3}=......\) (bn dùng máy tính)

b,xem lại chỗ 3.2

c,tính 4C , biến đổi tương tự câu a
 

1.2.3+2.3.4+...+48.49.50

=1.50

=50

\(C=1.2.3+2.3.4+...+48.49.50\)

\(\Rightarrow4C=1.2.3.4+2.3.4.4+...+48.49.50.4\)

\(=1.2.3.4+2.3.4.\left(5-1\right)+...+48.49.50.\left(51-47\right)\)

\(=1.2.3.4+2.3.4.5-1.2.3.4+...+48.49.50.51-47.48.49.50\)

\(=48.49.50.51\)

\(\Rightarrow C=\frac{48.49.50.51}{4}=1499400\)

A = 1.2 + 2.3 + 3.4 + ... + 1999.2000

3A = 1.2.3 + 2.3.3 + 3.4.3 + ... + 1999.2000.3

3A = 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 1999.2000.(2001 - 1998)

3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 1999.2000.2001 - 1998.1999.2000

3A = 1999.2000.2001

A = 1999.2000.2001 : 3 

A = 2 666 666 000