B=1.2.3+2.3.4+4.5.6+....+20.21.22

=>4B=4(1.2.3+2.3.4+3.4.5+...+20.21.22)

=>4B=1.2.3.4+2.3.4.4+3.4.5.4+...+20.21.22.4

=>4B=1.2.3.4+2.3.4.(5-1)+3.4.5.(6-2)+...+20.21.22(23-19)

=>4B=1.2.3.4+2.3.4.5-2.3.4+3.4.5.6-2.3.4.5+...+20.21.22.23-19.20.21.22

=>4B=20.21.22.23

=>4B=212520

=>B=53130

mình nha

Đặt A = 1 . 2 . 3 + 2 . 3 . 4 + 3 . 4 . 5 + 4 . 5 . 6 + ... + 20 . 21 . 22

=> 4A = 1 . 2 . 3 . 4 + 2 . 3 . 4 . 4 + 3 . 4 . 5 . 4+ 4 . 5 . 6 . 4 + ... + 20 . 21 . 22 . 4

=> 4A = 1 . 2 . 3 . ( 4 - 0 ) + 2 . 3 . 4 . ( 5 - 1 ) + 3 . 4 . 5 . ( 6 - 2 ) + 4 . 5 . 6 . ( 7 - 3 ) + ... + 20 . 21 . 22 . ( 23 - 19 )

=> 4A = 1 . 2 . 3 . 4 - 0 + 2 . 3 . 4 . 5 - 1 . 2 . 3 . 4 + 3 .4 . 5 . 6 - 2 . 3 . 4 . 5 + 4 . 5 . 6 . 7 - 3 . 4 . 5 . 6 + ... + 20 . 21.22.23 - 19.20.21.22

=> 4A = 0 + 20 . 21 . 22 . 23 

=> 4A = 212520

=> A = \(\frac{212520}{4}=53130\)

Ta có:

B=1.2.3+2.3.4+3.4.5+...+20.21.22

=>4B=1.2.3(4-0)+2.3.4(5-1)+3.4.5(6-2)+...+20.21.22(23-19)

<=>4B=1.2.3.4+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+...+20.21.22.23-19.20.21.22

<=>4B=20.21.22.23

<=>B=5.21.22.23=53130

53130

\(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{20.21.22}=\frac{1}{2}\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}+...+\frac{2}{20.21.22}\right)\)

\(=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+\frac{1}{3.4}-\frac{1}{4.5}+....+\frac{1}{20.21}-\frac{1}{21.22}\right)\)

\(=\frac{1}{2}\left(\frac{1}{2}-\frac{1}{462}\right)=\frac{1}{2}.\frac{115}{231}=\frac{115}{462}\)

a) Số hạng trong tổng có dạng n.(n+1)(n+2) 

nhận xét: n(n+1)(n+2)(n+3) - (n-1).n(n+1)(n+2) = 4.n(n+1)(n+2). Tính A 

4.A = 2.3.4.(5-1) + 3.4.5.(6-2) + ...+ 20.21.22.(23 - 19)

4.A = 2.3.4.5 - 1.2.3.4 + 3.4.5.6 - 2.3.4.5 + ...+ 20.21.22.23 - 19.20.21.22

4.A = (2.3.4.5 + 3.4.5.6 + ...+ 20.21.22.23) - (1.2.3.4 + 2.3.4.5 + ...+ 19.20.21.22)

4.A = 20.21.22.23 - 1.2.3.4 = 212 496 => A = 53 124

b) Em xem lại : dạng nào đã hỏi rồi , em nên tự làm 

Đặt A=1.2.3+2.3.4+.....+5.6.7

4A=1.2.3.4+2.3.4.(5-1)+.........+5.6.7.(8-4)

4A=1.2.3.4+2.3.4.5-1.2.3.4+........+5.6.7.8-4.5.6.7

4A=5.6.7.8

A=5.6.7.8:4

A=420

A=1(2+1)+2(3+1)+3(4+1)+...+99(100 +1 )

A=1.2+1+2.3+2+3.4+3...99.100+99

A=(1.2+2.3+3.4+...99.100)+(1+2+3+4...99)

giải:

Đặt A=1.2.3+2.3.4+3.4.5+4.5.6+...+98.99.100

4A=(1.2.3+2.3.4+3.4.5+4.5.6+...+98.99.100)4

4A=1.2.3(4-0)+2.3.4(5-1)+3.4.5(6-2)+4.5.6(7-3)+...+98.99.100(101-97)

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+...+98.99.100.101-97.98.99.100

4A=1.2.3.4-1.2.3.4+2.3.4.5-2.3.4.5+3.4.5.6-3.4.5.6+...+97.98.99.100-97.98.99.100+98.99.100.101

4A=98.99.100.101

=>A=98.99.100.101/4

A=1.2.3+2.3.4+...+2416.2417.2418=(2416.2417.2418.2419):4
B=1.2.3-2.3.4+...+2415.2416.2417-2416.2417.2418
A+B=2(1.2.3+3.4.5+...+2415.2416.2417)=2C
Xét C=1.2.3+3.4.5+...+2415.2416.2417
=1.3(5-3)+3.5(7-3)+...+2415.2417.(2419-3)
=1.3.5+3.5.7+...+2415.2417.2419-3(1.3+3.5+...2415.2417)
=(1.3.5.7+3.5.7.(9-1)+...+2415.2417.2419.(2421-2413)):8-3.(1.3+1.3.5+3.5.(7-1)+...+2415.2417(2419-2413)):6
=2415.2417.2419.2421:8-3.(1.3+2415.2417.2419):6
=> B=2C-A
Ko chắc lắm

dê et ??? bieu thuc nay ko co lien quan đê gia tri cua thua so h cua 1 so