Question

Tính: S=1x2+2x3+....+38x39+39x40

Answer 1

s=2-1+3-2+...+39-38+40-39

  =40-1

 = 39

Answer 2

thank you

Answer 3

\(S=1.2+2.3+..........+39.40\)

\(3S=1.2.3+2.3.3+...........+39.40.3\)

\(3S=\left(3-0\right).1.2+\left(4-1\right).2.3.+......+\left(41-38\right).39.40.\)

\(3S=1.2.3-0.1.2+2.3.4-1.2.3+......+39.40.41-38.39.40\)

\(3S=0+39.40.41\)

\(3S=63960\)

   \(S=63960:3=21320\)

Answer 4

kết quả của nghĩa là sai