I'm tackling ps10 now.
Since I'm not very good to read English, there is some passages which I can't interpret properly.

What does it mean, written in problem:1?
"Σ (distance of point to the centroid of its encapsulating cluster)**2 where the sum is over all points in the training set."

1.sum of what ?
2.does "sum is over all points in the training set" means, sum of something is over sum of all distances of points in the training set ?

I hope I could hold the meaning reversely from the solution, but the attached solution to this problem is wrong. (it should be solution of ps9.)

Question

I'm tackling ps10 now. 
Since I'm not very good to read English, there is some passages which I can't interpret properly.

What does it mean, written in problem:1?
"Σ (distance of point to the centroid of its encapsulating cluster)**2 where the sum is over all points in the training set."

1.sum of what ? 
2.does "sum is over all points in the training set" means, sum of something is over sum of all distances of points in the training set ?

I hope I could hold the meaning reversely from the solution, but the attached solution to this problem is wrong. (it should be solution of ps9.)