P is the pre stressed force applied to tendon of a rectangular pre stressed beam whose area of cross section is A and sectional modulus is Z. The minimum stress f on the beam subjected to a maximum bending moment M is
Af=(P/A) - (Z/M)
Bf = (A/P) - (M/Z)
Cf=(P/A) - (M/Z)
Df=(P/A) - (Z/6M)