Question

Cho A=1*2+2*3+3*4+...+19*20

     Tính A*3=?

Answer 1

A=1*2+2*3+3*4+....+19*20

=>A*3=1*2*3+2*3*3+3*3*4+....+3*19*20

=1+2+3-0*1*2+2*3*4-1*2*3+...+19*20*21-18*19*20

=19*20*21

=7980

Answer 2

A=1x2+2x3+3x4+........+19x20

\(\Rightarrow\)Ax3=1x2x3+2x3x3+3x3x4+..........+3x19x20

=1x2x3-0x1x2+2x3x4-1x2x3+3x4x5-2x3x4+.......+19x20x21-18x19x20

=19x20x21

=7980