--- loncom/homework/functionplotresponse.pm	2010/11/11 03:52:35	1.33
+++ loncom/homework/functionplotresponse.pm	2010/11/11 15:57:49	1.34
@@ -1,7 +1,7 @@
 # LearningOnline Network with CAPA
 # option list style responses
 #
-# $Id: functionplotresponse.pm,v 1.33 2010/11/11 03:52:35 www Exp $
+# $Id: functionplotresponse.pm,v 1.34 2010/11/11 15:57:49 www Exp $
 #
 # Copyright Michigan State University Board of Trustees
 #
@@ -639,14 +639,10 @@ sub populate_arrays {
                    my $dxdt=&ddt_cubic_hermite($t,@xparms);
                    if ($dxdt) {
                       $Apache::functionplotresponse::dfuncdx[$xi]=&ddt_cubic_hermite($t,@yparms)/$dxdt;
-                   }
-# Faa di Bruno
-# d^2y/dx^2=(d^2y/dt^2)/(dx/dt)^2+(dy/dt)/(d^2x/dt^2)
-                   my $d2xdt2=&d2dt2_cubic_hermite($t,@xparms);
-                   if (($dxdt) && ($d2xdt2)) {
+# Second derivative
                       $Apache::functionplotresponse::d2funcdx2[$xi]=
-                         &d2dt2_cubic_hermite($t,@yparms)/($dxdt*$dxdt)
-                        +&ddt_cubic_hermite($t,@yparms)/$d2xdt2;
+                         ($dxdt*&d2dt2_cubic_hermite($t,@yparms)-&ddt_cubic_hermite($t,@yparms)*&d2dt2_cubic_hermite($t,@xparms))/
+                         ($dxdt*$dxdt*$dxdt);
                    }
                 }
             }