Limit of (1/n)*((x+a/n)^2+(x+2a/n)^2+...+(x+(n-1)a/n)^2) as n -> infinity

[vinhtq115]

Previously, I have solved a similar one: lim[i -> infty] 1/n*[(x+a/n)+(x+2*a/n)+...+(x+(n-1)*a/n)] but I can't do this one:
lim[i -> infty] 1/n*[(x+a/n)^2+(x+2*a/n)^2+...+(x+(n-1)*a/n)^2]
I'm getting stuck and don't know how to solve this part: [(x+a/n)^2+(x+2*a/n)^2+...+(x+(n-1)*a/n)^2]

 

[Deleted member 4993]

Previously, I have solved a similar one: lim[i -> infty] 1/n*[(x+a/n)+(x+2*a/n)+...+(x+(n-1)*a/n)] but I can't do this one:
lim[i -> infty] 1/n*[(x+a/n)^2+(x+2*a/n)^2+...+(x+(n-1)*a/n)^2]
I'm getting stuck and don't know how to solve this part: [(x+a/n)^2+(x+2*a/n)^2+...+(x+(n-1)*a/n)^2]

What do you get when you differentiate your new series?