Since the tooth profile of the rack straight tooth (β = 0) axis surface is a straight line parallel to the ZP axis, its equation has no practical use, so it is similar to section 1.4 4 do not derive its equation.
Because the tooth profile of straight rack tooth (= 0) is a straight line parallel to Zp axis, its equation has no practical use, so its equation is not derived here and in Section 1.4.4.