Expression 9: "d" equals StartRoot, left parenthesis, "x" Subscript, 2 , Baseline minus "x" Subscript, 1 , Baseline , right parenthesis squared plus left parenthesis, "y" Subscript, 2 , Baseline minus "y" Subscript, 1 , Baseline , right parenthesis squared , EndRootd=x2−x12+y2−y12