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You don't need to type the same formula on every row. Copy it down, and the spreadsheet adjusts the cell names for each row. D2's =B2*C2 becomes =B3*C3 on the next row, and so on.
To copy down, select the cell with the formula and the empty cells below it, then press Ctrl+D (Cmd+D on a Mac). You can also copy and paste, or drag the small square at the corner of the cell.