How To Use Variables In Document.evaluate()

I met a problem when using document.evaluate() to get text content
using XPath, my code is as follows:

nodes = document.evaluate("/html/body/div/ul[1]/li[5]", document,
null,XPathResult. UNORDERED_NODE_SNAPSHOT_TYPE , null);

in the XPath expression "/html/body/div/ul[1]/li[5]", I need to
traverse from li[1] to li[5],
so I should use a variable here , like this:

for (index =1; index<6; i++)
{
nodes = document.evaluate("/html/body/div/ul[1]/li[index]", ....);
// the variable index here is invalid!
}
my question is: How to use the variable in XPath expression?