Tìm n biết: (2n + 1) + (2n + 2) + (2n + 3) +...+ (2n + n) = 2265

1) Tìm n
a) (2n+1)+(2n+2)+(2n+3)+...+(2n+n)=2265
b) $\frac{n-1}{n}+\frac{n-2}{n}+\frac{n-3}{n}+...+\frac{1}{n}=2007$
2) Cho Cấp cố cộng $\left(u_n\right)$ biết $\left\{\begin{array}{l}{u}_1=4\\ d=3\end{array}\right.$ Tính
$S= u_1-u_3+u_5-u_7+...+u_{37}-u_{39}$