how calculate sum(x*y)

Hello All,

how calculate sum(x*y) ??

IF i try use sum() it say that need only node-list as argument....

I have xml
<root>
  <data>
   <x>2</x>
   <y>3</y>
  </data>
  <data>
   <x>5</x>
   <y>6</y>
  </data>
  .....
<root>


-- 
Best regards,
 Andrey Solo                          
mailto:solo(_at_)ibis(_dot_)odessa(_dot_)ua



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