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

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

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

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

\(=48.49.50.51\)

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

Ô tô đi với vận tốc 50km/giờ vì :

         100 : 2 = 50

                   đs : 50

Mình nghĩ đề bài thế này mới đúng : 1.2.3+2.3.4+...+48.49.50

Đặt B = 1.2.3 + 2.3.4 + ....... + 48.49.50

B = 1.2.3 + 2.3.4 +....+ 48.49.50
\(\Rightarrow\)4B= 1.2.3.4 + 2.3.4.4 +.....+48.49.50.4
=1.2.3.4 + 2.3.4.(5-1) +.....+48.49.50.(51-47)
=1.2.3.4 + 2.3.4.5 - 1.2.3.4 +.....+ 48.49.50.51 - 47.48.49.50
=48.49.50.51

= 1499400

Vậy 1.2.3 + 2.3.4 + ....... + 48.49.50 = 1499400

Cảm ơn bạn

Đặt A=1.2.3+2.3.4+3.4.5+........+48.49.50

     4A=1.2.3.4+2.3.4.4+..........+48.49.50.4

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

         =1.2.3.4+2.3.4.5-1.2.3.4+...........+48.49.50.51-47.48.49.50

         =48.49.50.51

         =5997600

       A=1499400

Vậy A=1499400

Đặt \(A=1\cdot2\cdot3+2\cdot3\cdot4+........+48\cdot49\cdot50\)

\(\Rightarrow4A=1\cdot2\cdot3\cdot4+2\cdot3\cdot4\cdot4+......+48\cdot49\cdot50\cdot4\)

\(=1\cdot2\cdot3\cdot4+2\cdot3\cdot4\cdot\left(5-1\right)+..........+48\cdot49\cdot50\cdot\left(51-47\right)\)

\(=1\cdot2\cdot3\cdot4+2\cdot3\cdot4\cdot5-1\cdot2\cdot3\cdot4+......+48\cdot49\cdot50\cdot51-47\cdot48\cdot49\cdot50\)

\(=48\cdot49\cdot50\cdot51\)

\(\Rightarrow A=\frac{48\cdot49\cdot50\cdot51}{4}\)

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