Question

[Nguyen Phuc]

Chứng minh mọi số nguyên dương n

1,4 + 2,7 + ... + n(3n+1) = n(n +1)²

Answer 1

[Nguyễn Việt Lâm]

\(\sum n\left(3n+1\right)=\sum3n^2+\sum n=3\sum n^2+\sum n\)

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

Answer 2

[Nguyen Phuc]

làm thep pp quy nạp mà b