Question

Tính S=1X2+2X3+3X4...+38X39+39X40.Trả lời S

Answer 1

S = 1.2+2.3+3.4+....+38.39+39.40

S.3 = ( 1.2+2.3+3.4+...+38.39+39.40 ) . 3

S.3 = 1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...........+38.39.(40-37)+39.40.(41-38)

S.3 = 1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+...........+38.39.40-37.38.39+39.40.41-38.39.40

S = \(\frac{39.40.41}{3}\)

S = 13.40.41

S = 21320