Đặt A = 1×2 + 2×3 + 3×4 + ... + 20×21
3A = 1×2×3 + 2×3×(4-1) + 3×4×(5-2) + ... + 20×21×(22-19)
= 1×2×3 - 1×2×3 + 2×3×4 - 2×3×4 + 3×4×5 - ... - 19×20×21 + 20×21×22
= 20×21×22
A = 20×21×22 : 3
= 20×22×7
= 3080
Tính tổng: 1x2 + 2x3 + 3x4 +...+ 2019x2020 + 2020x2021
tính nhanh (1x2)^-1+(2x3)^-1+(3x4)^-1+...+(2014x2015)^-1
tính A = 1x2 + 2x3 +3x4 +... +n x (n +1)
(1/(1x2)/(2x3)/(3x4)):(1/(2x3)/(3x4)/(4x5)):...(1/(97*98)/(98*99)/(99*100))
(1/(1x2)/(2x3)/(3x4)):(1/(2x3)/(3x4)/(4x5)):...(1/(97*98)/(98*99)/(99*100
1/(1x2)/(2x3)/(3x4)):(1/(2x3)/(3x4)/(4x5)):...(1/(97*98)/(98*99)/(99*100
(1/(1x2)/(2x3)/(3x4)):(1/(2x3)/(3x4)/(4x5)):...(1/(97*98)/(98*99)/(99*100
1/(1x2)/(2x3)/(3x4)):(1/(2x3)/(3x4)/(4x5)):...(1/(97*98)/(98*99)/(99*100
(1/(1x2)/(2x3)/(3x4)):(1/(2x3)/(3x4)/(4x5)):...(1/(97*98)/(98*99)/(99*100
ai lam dc mik tick cho
