Adding data of cells in one column and creating the additions in another

Let me explain what I am trying to achieve and I would totally appreicate any help:

I have a column with, say, these numbers: a1: 1 a2: 3 a3: 4.5 a4: 9

I want to be able to create a parallel column where I will have: b1 : 1 b2: 4 b3: 8.5 b4: 17.5

note that the second column is just adding numbers of all the prev. cells of column a: a1, then a1+a2, a1+a2+a3 etc.

Hope my question is clear!

Thank you so much,

Dana Berliner

Put this in B1 and copy down:

=SUM($A$1:A1)