Question

Tinh S =1.2+2.3+4.3+.........+39.40

Answer 1

=39 . 40 . 41 / 3

Answer 2

3S=1.2.3+2.3.3+...+39.40.3

3S=1.2.3+2.3.(4-1)+...+39.40.(41-38)

3S=1.2.3-1.2.3+2.3.4-...-38.39.40+39.40.41

3S=39.40.41

S=13.40.41

S=21320