[Solved] operations with javascript in html document

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You are getting NaN because of this:

var altezza = document.getElementById('altezza');
var base = document.getElementById('base');
(base*altezza/2)

getElementById() does exactly that – gets the element. You just need to get the value from the element e.g. var altezza = document.getElementById('altezza').value;

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solved operations with javascript in html document