avoid recomputation.

f_a/g_a has already been computed and we can use it directly.

Change-Id: I39b07205c7d2486e88076089fa960f116919f27f

include/ceres/jet.h

diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index f6fe4ad..082bcc3 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -391,7 +391,7 @@
   // which holds because v*v = 0.
   const T g_a_inverse = T(1.0) / g.a;
   const T f_a_by_g_a = f.a * g_a_inverse;
-  return Jet<T, N>(f.a * g_a_inverse, (f.v - f_a_by_g_a * g.v) * g_a_inverse);
+  return Jet<T, N>(f_a_by_g_a, (f.v - f_a_by_g_a * g.v) * g_a_inverse);
 }
 
 // Binary / with a scalar: s / x