Question

[kim anh]

Tính A=1.2+2.3+3.4+...+n.(n+1)

giúp mk đi m.n..

Answer 1

[๖ۣۜNɦσƙ ๖ۣۜTì]

A =1.2+2.3+3.4+.............+n(n+1)
   =1(1+1) + 2(2+1) + 3(3+1) +...+n(n+1)
   =(1^2 + 2^2 + 3^2 +...+ n^2) + (1 + 2 + 3 + ...+ n)
Ta có các công thức:
1^2 + 2^2 + 3^2 +...+ n^2 = n(n+1)(2n+1)/6
1 + 2 + 3 + ...+ n = n(n+1)/2
Thay vào ta có:
S = n(n+1)(2n+1)/6 + n(n+1)/2
=n(n+1)/2[(2n+1)/3 + 1]
=n(n+1)(n+2)/3

Answer 2

[ミ★๖ۣۜSεηαbĭ ๖ۣۜTĭʂт ๖ۣۜKεү ★彡]

\(A=1.2+2.3+3.4+...+n\left(n+1\right)\)

\(3A=1.2.3+2.3.4+3.4.3+..+3n\left(n+1\right)\)

\(=1.2.3+2.3\left(4-1\right)+3.4\left(5-2\right)+...+n\left(n+1\right)\left[\left(n+2\right)-\left(n-1\right)\right]\)

\(=n\left(n+1\right)\left(n+2\right)\)

\(=\frac{n\left(n+1\right)\left(n+2\right)}{3}\)

ko chắc vì mk làm qua lâu òi hc tốt ~~:B~~