A=4+12+24+40+...........+19404+19800 

=2(1.2+2.3+3.4+4.5+...+98.99+99.100) 

Xét tổng: B=1.2+2.3+3.4+4.5+...+98.99+99.100 

=> 3B= 1.2.3 + 2.3.3 + 3.4.3 + 4.5.3 +...+ 98.99.3 + 99.100.3 

= 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + 4.5.(6 - 3)+...+ 98.99.(100 - 97) + 99.100.(101 - 98) 

= 1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6- 3.4.5+...+ 98.99.100-97.98.99+99.100.101-98.99.100 

= 99.100.101 

=999900 => B=333300 

A= 2B = 333300.2 = 666600 

A=4+12+24+40+...+19404+19800

1/2A=2+6+12+...+9702+9909

1/2A=1.2+2.3+3.4+....+98.99+99.100

3/2A=1.2.3 + 2.3.(4-1) +3.4.(5-2)+...+98.99.(100-97) + 99.100.(101-98)

3/2A=1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4...+98.99.100 -97.98.99 + 99.100.101 -98.99.100

3/2A= 99.100.101

A=(99.100.101) : 3/2 = 666600

Vậy A = 666600                                                    mik thêm A vào cho bạn dễ hiểu

\(D=4+12+24+40+....+19404+19800\)

\(\Rightarrow\dfrac{1}{2}D=2+6+12+20+...+9702+9900\)

\(\Rightarrow\dfrac{1}{2}D=1.2+2.3+3.4+4.5+....+98.99+99.100\)

\(\Rightarrow3.\dfrac{1}{2}D=\dfrac{3}{2}D=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+4.5.\left(6-3\right)+...+98.99.\left(100-97\right)+99.100.\left(101-98\right)\)

\(\Rightarrow\dfrac{3}{2}D=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.99-99.100.101-98.99.100\)

\(\Rightarrow\dfrac{3}{2}D=99.100.101=999900\)

\(\Rightarrow D=999900:\dfrac{3}{2}=666600\)

Vậy \(D=666600\)

D= 4 + 12 + 24 +.... + 19404 + 19800

= 2 + 6 + 12 + ......+ 9702 + 9900

= 1.2 + 3.2 + 3.4 +.....+ 98.99 + 99.100

= 1.2.3 + 2.3.(4-1) + 3.4.(5-2) +......+ 98.99.(100-97) + 99.100.(101-98)

= 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4+......+ 98.99.100 + 99.100.101

= (99.100.101)

= 666600

(mình sợ bạn ko hiểu )

Chúc bạn làm bài tốt + zui zẻ

lớp 7 s em lớp 6 nè cũng câu tương tự khó wa bít rùi thì bảo em nhé
 

D=4+12+24+40+...+19404+19800

1/2D=2+6+12+...+9702+9900

1/2D=1.2+2.3+3.4+...+98.99+99.100

3/2D=1.2,3+2.3.(4-1)+3.4.(5-2)+...+98.99.(100-97)+99.100.(101-98)

3/2D=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.99+99.100.101-98.99.100

3/2D=99.100.101

D=(99.100.101):3/2=666600