Question

A= 1x2+ 2x3+ 3x4+ ....+18x20

Answer 1

sai một chỗ

\(A=1.2+2.3+3.4+...+18.19\)

\(\Leftrightarrow3A=1.2.3+2.3.3+3.4.3+...+18.19.3\)

\(\Leftrightarrow3A=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+18.19.\left(20-17\right)\)

\(\Leftrightarrow3A=1.2.3+2.3.4-1.2.3+...+18.19.20-17.18.19\)

\(\Leftrightarrow3A=18.19.20\)

\(\Leftrightarrow A=6.19.20\)

Answer 2

A= 1.2 + 2.3 + 3.4 + ... + 18.20

3A= 1.2.3 + 2.3.3 + 3.4.3 + ... + 18.20.3

3A= 1.2.3 + 2.3.(4-1) + 3.4.(5-2) + ... + 18.20.(21-18)

3A= 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 18.20.21 - 18.19.20

3A= 18.20.21

A= 2520

Answer 3

ko mình thi h/s giỏi toán mà đề cho là thế nên