Expression 8: "A" Subscript, "C" "D" , Baseline equals tangent to the negative 1st power StartFraction, "A" Subscript, 1 , Baseline minus "C" Subscript, 1 , Baseline Over "C" Subscript, 2 , Baseline minus "A" Subscript, 2 , Baseline , EndFraction plus tangent to the negative 1st power StartFraction, "C" Subscript, 1 , Baseline minus "D" Subscript, 1 , Baseline Over "C" Subscript, 2 , Baseline minus "D" Subscript, 2 , Baseline , EndFractionACD=tan−1A1−C1C2−A2+tan−1C1−D1C2−D2