How to get the position of element transformed with css rotate

Per your current Question and your requested confirmation of:

var x = termin.top + Math.cos(angle) * div.height;var y = div.left + Math.sin(angle) * div.height;

The solution can be found in this other SO Answer for a different question, enhanced here:

// return an object with full width/height (including borders), top/bottom coordinatesvar getPositionData = function(el) {    return $.extend({        width: el.outerWidth(false),        height: el.outerHeight(false)    }, el.offset());};// get rotated dimensions   var transformedDimensions = function(el, angle) {    var dimensions = getPositionData(el);    return {        width: dimensions.width + Math.ceil(dimensions.width * Math.cos(angle)),        height: dimensions.height + Math.ceil(dimensions.height * Math.cos(angle))    };};

Here's an interactive jsFiddle that provides real-time updates for getPositionData(); function.
You'll be able to see the top and left values at the end of the CSS3 Rotation process you control.

Reference:   jsFiddle

Status Update: The above jsFiddle works great for 0-90deg and can be approved upon for all angles and different units such as rad, grad, and turn.

Try using element.getBoundingClientRect()

According to MDN:

Starting in Gecko 12.0 (Firefox 12.0 / Thunderbird 12.0) , the effect of CSS transforms is considered when computing the element's bounding rectangle.