Question

 

Tính S bằng 1x2+2x3+3x4+...+38x39+39X40

tÌM s

Answer 1

\(S=1.2+2.3+3.4+...+38.39+39.40\)

\(3S=1.2.3+2.3.\left(4-1\right)+...+38.39.\left(40-37\right)+39.40.\left(41-38\right)\)

\(3S=1.2.3+2.3.4-1.2.3+...+38.39.40-37.38.39+39.40.41-38.39.40\)

\(3S=39.40.41\)

\(S=\frac{39.40.41}{3}\)

\(S=21320\)

Vậy \(S=21320\)

Tham khảo nhé~

Answer 2

ko hiểu.