\(A=1.2+2.3+........+19.20\)
\(3A=3.1.2+3.2.3+.........+3.19.20\)
\(3A=\left(3-0\right).1.2+\left(4-1\right).2.3+..........+\left(21-18\right).19.20\)
\(3A=1.2.3-0.1.2+2.3.4-1.2.3+........+19.20.21-18.19.20\)
\(3A=19.20.21\)
\(3A=7980\)
Chú thích : dấu . là dấu nhân nha