Question

1.3 + 2.5 +3.7 + ... + n(2n + 1)=n÷6(n+1)(4n+5)​

Answer 1

Answer:

1.3+2.5+3.7+4.9+n(2n+1)=n÷6(n+1)(4n+5)

Answer 2

Step-by-step explanation:

$\sum \: r(2r + 1) = 2\sum \: {r}^{2} + \sum \: r \\ = 2. \frac{n(n + 1)(2n + 1)}{6} + \frac{n(n + 1)}{2} \\ = \frac{ n(n + 1)}{6}.(4n + 2 + 3) \\ = \frac{n(n + 1)(4n + 5)}{6}$