(* Created with the Wolfram Language for Students - Personal Use Only : www.wolfram.com *)
-(CA^2*(-2 + D)*z3*(-(s34*(sP4*z3 + sP3*(z3 - 2*z4))) + s34^2*(z3 + 2*z4) + 
    2*sP3*(-(sP4*z3) + sP3*z4)))/(2*s34^2*sP3*(-1 + z34)*z34)
