use ._scaling_factor() instead of .formal()[1]

Author: yyyyx4

src/sage/schemes/elliptic_curves/ell_curve_isogeny.py

@@ -3346,7 +3346,7 @@ def dual(self):
 
         # trac 7096
         # this should take care of the case when the isogeny is not normalized.
-        u = self.formal(prec=2)[1]
+        u = self._scaling_factor()
         isom = WeierstrassIsomorphism(E2pr, (u/F(d), 0, 0, 0))
 
         E2 = isom.codomain()
@@ -3369,7 +3369,7 @@ def dual(self):
         # the composition has the degree as a leading coefficient in
         # the formal expansion.
 
-        phihat_sc = phi_hat.formal(prec=2)[1]
+        phihat_sc = phi_hat._scaling_factor()
 
         sc = u * phihat_sc/F(d)
 

src/sage/schemes/elliptic_curves/hom.py

@@ -394,8 +394,7 @@ def is_normalized(self):
             sage: phi.is_normalized()
             True
         """
-        phi_formal = self.formal(prec=5)
-        return phi_formal[1] == 1
+        return self._scaling_factor() == 1
 
 
     def is_separable(self):