The covalent bond length of the AB molecule is the internuclear distance between atom A and atom B.
Bond length = rA+rB
The total length of the AB molecule is the distance between the outermost boundaries of the two atoms along the internuclear axis.
Total length = Radius of A + Bond length + Radius of B
Total length = rA+(rA+rB)+rB=2(rA+rB)
Answer: (rA+rB),2(rA+rB)