ARRAYFORMULA: one formula per column

In Level 1 you used ARRAYFORMULA to do math on whole ranges. Here's why it matters in shared sheets: a column with one formula at the top can't be half-copied, half-overwritten or forgotten on row 212.

The pattern you'll use most guards against empty rows:

=ARRAYFORMULA(IF(A2:A="", "", B2:B*C2:C))

A2:A means "from A2 to the bottom", so new rows are covered the moment someone types them. The IF keeps empty rows blank instead of filling the sheet with zeros.

Two gotchas:

• AND and OR don't work row by row. AND(range1>0, range2>0) collapses everything into one TRUE or FALSE. Multiply the conditions for AND, (B2:B>0)*(C2:C>0), and add them for OR.
• Some functions already spill. FILTER, SORT, UNIQUE and QUERY don't need ARRAYFORMULA around them.