Question

Tính giá trị biểu thức: B= 1.2.3 + 2.3.4 + 3.4.5 + ... + 17.28 19

Answer 1

Bùi Lê Anh Khoa

Ta có: B = 1.2.3 + 2.3.4 + 3.4.5 + ... + 17.18.19
=> 4B = 4(1.2.3 + 2.3.4 + 3.4.5 + ... + 17.18.19)
=> 4B = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 +...... +17.18.19.4
=> 4B = 1.2.3.4 + 2.3.4(5 - 1) + 3.4.5.(6 - 2) +..... +17.18.19.(20 - 16)
=> 4B = 1.2.3.4 + 2.3.4.5 - 2.3.4 + 3.4.5.6 - 2.3.4.5 + ..... + 17.18.19.20 - 16.17.18.19
=> 4B = 17.18.19.20
=> 4B = 116280
=> B = 29070

Answer 2

Ta có: B = 1.2.3 + 2.3.4 + 3.4.5 + ... + 17.18.19
=> 4B = 4(1.2.3 + 2.3.4 + 3.4.5 + ... + 17.18.19)
=> 4B = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 +...... +17.18.19.4
=> 4B = 1.2.3.4 + 2.3.4(5 - 1) + 3.4.5.(6 - 2) +..... +17.18.19.(20 - 16)
=> 4B = 1.2.3.4 + 2.3.4.5 - 2.3.4 + 3.4.5.6 - 2.3.4.5 + ..... + 17.18.19.20 - 16.17.18.19
=> 4B = 17.18.19.20
=> 4B = 116280
=> B = 29070

Answer 3

B=1.2.3 + 2.3.4 + 3.4.5 + ... + 17.18.19

4B=1.2.3.4+2.3.4.(5-1)+3.4.5.(6-2)+...+17.18.19.(20-16)

4B=1.2.3.4+ 2.3.4.5 - 2.3.4 + 3.4.5.6 - 2.3.4.5 +... + 17.18.19.20-16.17.18.19

4B=17.18.19.20

B=17.18.19.5

B=29070

P/S:Đề bạn sai mik chữa lại rồi đó.