Question

tính tổng: 1.2 + 2.3 +...+ n.(n+1)

Answer 1

A =1.2 + 2.3 + ....+ n.(n+1)

A = n(n+1) + ....+ 2.3 + 1.2

A\(\times\) 3 = n(n+1).3 +....+ 2.3.3+ 1.2.3

A\(\times\)3 = n(n+1)[n+2 - (n -1)]+....+2.3.(4-1) +1.2.3

A\(\times\)3 = n(n+1)(n+2) - (n-1)n(n+1) +....+ 2.3.4 - 1.2.3 + 1.2.3

A\(\times\)3 = n(n+1)(n+2)

A \(\times\)3 = n(n+1)(n+2)

A = n(n+1)(n+2) : 3

 

 

Answer 2

A =1.2 + 2.3 + ....+ n.(n+1)

A = n(n+1) + ....+ 2.3 + 1.2

A$\times$ 3 = n(n+1).3 +....+ 2.3.3+ 1.2.3

A$\times$3 = n(n+1)[n+2 - (n -1)]+....+2.3.(4-1) +1.2.3

A$\times$3 = n(n+1)(n+2) - (n-1)n(n+1) +....+ 2.3.4 - 1.2.3 + 1.2.3

A$\times$3 = n(n+1)(n+2)

A $\times$3 = n(n+1)(n+2)

A = n(n+1)(n+2) : 3

Ht!