repeat operation and add to total in one cell
https://docs.google.com/spreadsheets/d/1Kkz8rpI_4ZpxUmKxXoMZkR4w6OSLe9PAg_k402It85A/edit?usp=sharing
i want the selected cell, B9 (and the other cells below it but if i figure out the formula for one its easy enough to apply) to display the total of a series of operations. Each operation is taking a value in the row and multiplying it by the cell that's in the same column in row 7. I've typed out the long way to do it, "=(C9*$C7)+(D9*$D$7)+(E9*$E$7)+(F9*$F$7)+(G9*$G$7)+(H9*$H$7)" in this case, but i need a solution that i can type out once and will automatically update as more columns are added to the right. The full version of this spreadsheet has hundreds of rows and dozens of columns with more to be added with time, which in terms of the effort I'm expending rules out both doing it manually as more columns are added and creating a secondary cell for each of the cells to store the result of each operation to be summed.
Thanks for the help!