Thread: Series and Sequences help please..

if Σ of r=1 to n of U_a is equal to 3n^2+2n , find U_a. Hence find Σ of r=n+1 to 2n of U_a Ans: 6r-1 , 9n^2+2n

and also find sum of: Σ of r=n to 2n of r(2r+3) ans1/6)n(28n^2+57n+29)
: Σ of r=1 to 3n of r(r^2+1) ans3/4)n(3n+1)(9n^2+3n+2)

and can you give tips on how to find the sum of the series if this case come across...
Σ of r=n to 2n and also of the r is as usual =1 but to 3n