Expression 28: "t" Subscript, 1 , Baseline equals StartFraction, negative "r" Subscript, 1 , Baseline "i" Subscript, 1 , Baseline minus "r" Subscript, 2 , Baseline "i" Subscript, 2 , Baseline minus "r" Subscript, 3 , Baseline "i" Subscript, 3 , Baseline minus "d" Over "r" Subscript, 1 , Baseline "r" Subscript, 1 , Baseline plus "r" Subscript, 2 , Baseline "r" Subscript, 2 , Baseline plus "r" Subscript, 3 , Baseline "r" Subscript, 3 , Baseline , EndFractiont1=−r1i1−r2i2−r3i3−dr1r1+r2r2+r3r3