Question

tính tổng:

S= 1x2+2x3+3x4+4x5+...+ 20x21

Answer 1

S= 1x2+2x3+3x4+4x5+...+ 20x21

3xS=3x( 1x2+2x3+3x4+4x5+...+ 20x21 )

3xS = 1x2x3+2x3x3+3x4x3+....+20x21x3

3xS = 1x2x3 + 2x3x(4-1) + 3x4x(5-2)+........+20x21x(22-19)

3xS= 1x2x3 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4 +......+20x21x22 - 19x20x21

3xS = 20x21x22

 S = 20x21x22 /3

     

Answer 2

S= 1x2+2x3+3x4+4x5+...+ 20x21

3xS=3x( 1x2+2x3+3x4+4x5+...+ 20x21 )

3xS = 1x2x3+2x3x3+3x4x3+....+20x21x3

3xS = 1x2x3 + 2x3x(4-1) + 3x4x(5-2)+........+20x21x(22-19)

3xS= 1x2x3 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4 +......+20x21x22 - 19x20x21

3xS = 20x21x22

S = 20x21x22 /3

k mk nha