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

TICK ĐÚNG GIÚP MÌNH Ặ

T/c:A=1/1*2*3+1/2*3*4+1/3*4*5+1/4*5*6+...+1/97*98*99+1/98*99*100

2A=2/1*2*3+2/2*3*4+2/3*4*5+2/4*5*6+...+2/97*98*99+1/98*99*100

2A=(1/1*2-1/2*3)+(1/2*3-1/3*4)+(1/3*4-1/4*5)+.....+(1/97*98-1/98*99)+(1/98*99-1/99*100)

2A=1/2+1/99*100

A=tự tính nha

A= [(1/2-1/2*3)/2]+[(1/2-1/3*4)/2]+...+[(1/2-1/99*100)/2]

A=(1/2-1/99*100)/2

A=-101/198/2

A=-101/396

[1.2.3+98.99.100]x2+1=bạn tự tính nha
 

đây là bài lớp 5
 

OH MY GOD ! Bam may tinh moi het ca tay

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

4A = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 + ... + 98.99.100.4

4A = 1.2.3.4 + 2.3.4.(5 - 1) + 3.4.5.(6 - 2) + ... + 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 + ... + 98.99.100.101 - 97.98.99.100

4A = 98.99.100.101

=> A = 98.99.100.101 : 4

=> A = 24497550

\(B=1\cdot2\cdot3+2\cdot3\cdot4+3\cdot4\cdot5+...+98\cdot99\cdot100\)

\(\Rightarrow4B=4\cdot\left(1\cdot2\cdot3+2\cdot3\cdot4+...+98\cdot99\cdot100\right)\)

\(\Rightarrow4B=1\cdot2\cdot3\cdot\left(4-0\right)+2\cdot3\cdot4\cdot\left(5-1\right)+3\cdot4\cdot5\cdot\left(6-2\right)+...+98\cdot99\cdot100\cdot\left(101-97\right)\)

\(\Rightarrow4B=1\cdot2\cdot3\cdot4+2\cdot3\cdot4\cdot5-1\cdot2\cdot3\cdot4-....+98\cdot99\cdot100\cdot101-97\cdot98\cdot99\cdot100\)

\(\Rightarrow4B=98\cdot99\cdot100\cdot101\)

\(\Rightarrow B=\dfrac{98\cdot99\cdot100\cdot101}{4}\)

\(\Rightarrow B=25\cdot98\cdot99\cdot101\)

B=1x2x3+2x3x4+...+98x99x100

=>4B=1x2x3x(4-0)+2x3x4x(5-1)+...+98x99x100x(101-97)

4B=1x2x3x4+2x3x4x5-1x2x3x4+...+98x99x100x101-97x98x99x100

4B=98x99x100x101

=>B=\(\dfrac{98\cdot99\cdot100\cdot101}{4}\)=24497550.

A= 1.2.3 +2.3.4 + 3.4.5 + ... + 97.98.99

=> 4A = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 + ... + 97.98.99.4

=> 4A =1.2.3.4 + 2.3.4.(5-1) + 3.4.5(6-2) + ...+ 97.98.99( 100 - 96)

=> 4A = 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + 3.4.5.6 - 2.3.4.5 + 97.98.99.100 - 96.97.98.99.

=>4A= 97.98.99.100

=> A= (97.98.99.100)/ 4 = 97.98.99.25

Em có thể tham khảo cách làm tương tự như link: 

Cách làm nhé. Đừng chép hết. Đề bài của bạn khác 1 chút so với của em.

Câu hỏi của Ngô Hồng Thuận - Toán lớp 7 - Học toán với OnlineMath

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

4A = 98.99.100.4 + .....+ 3.4.5.4 + 2.3.4.4 + 1.2.3.4

4A = 98.99.100.(101-97) +... + 2.3.4.(5-1) + 1.2.3.4

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

4A = 98.99.100.101

  A = 98.99.100.101 : 4

  A = 24497550

Chắc thế!

Ta có: \(S=1\cdot2\cdot3+2\cdot3\cdot4+3\cdot4\cdot5+...+97\cdot98\cdot99\)

\(\Leftrightarrow4\cdot S=1\cdot2\cdot3\cdot\left(4-0\right)+2\cdot3\cdot4\cdot\left(5-1\right)+3\cdot4\cdot5\cdot\left(6-2\right)+...+97\cdot98\cdot99\cdot\left(101-97\right)\)

\(\Leftrightarrow4\cdot S=98\cdot99\cdot100\cdot101\)

\(\Leftrightarrow S=\text{24497550}\)

\(S=1.2.3+2.3.4+...+97.98.99\)

\(\Rightarrow4S=1.2.3.4+2.3.4\left(5-1\right)+...+97.98.99.\left(100-96\right)\)

\(\Rightarrow4S=1.2.3.4+2.3.4.5-1.2.3.4+...+97.98.99.100-96.97.98.99\)

\(\Rightarrow4S=97.98.99.100\)

\(\Rightarrow S=\frac{97.98.99.100}{4}\)

\(\Rightarrow S=97.98.99.25\)

S=1.2.3+2.3.3.4.5+...+97.98.99

4.S=1.2.3.4+2.3.4(5-1)+3.4.5.(6-2)+....+97.98.99.(100-96)

4S-S=1.2.3.4+2.3.4.(5-1)-1.2.3.4+3.4.5.(6-2)-2.3.4.(5-1)+...+97.98.99.(100-96)-96.97.98.(99-95)

S= 97.98.99.100:4=23 527 350