Question

Tính S=1.2+2.3+3.4+........+39.40

Answer 1

S = 333300

Answer 2

 S=1.2+2.3+3.4+...+39.40

3S=1.2.3+2.3.3+3.4.3+...+39.40.3

3S=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...+39.40.(41-38)

3S=1.2.3+1.2.0+2.3.4+2.3.1+3.4.5+3.4.2+...+39.40.41+39.40.38

3S=1.2.0+39.40.41= 63960

  S=63960 :3= 21320

          Vậy S= 21320