Ax3=1x2x3+2x3x3+...+19x20x3
=1x2x3+2x3x(4-1)+...+19x20x(21-18)
=1x2x3+2x3x4-1x2x3+....+19x20x21-18x19x20
=19x20x21=7980
A = 1 x 2 + 2 x 3 + 3 x 4 ... 19 x 20
=>A x 3=1x2x3+2x3x3+3x3x4+...+3x19x20
=1x2x3 - 0x1x2 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4+....+19x20x21 - 18x19x20
=19x20x21
=7980