Could you help me with JavaScript, XML, MathML, PHP, HTML project (little bit big project)

I'm having an issue with web application: electrotechnics calculator. I need to add to add to this application few things: functions (sin, cos, tg, cotg), and lower index. It is quite a challange because i think it is not easy... In XML is written, what will be shown on site (utvary.xml). In file called rovnice-resitel is tree, which is (changing) the equation like: U=R*I... I=U/R.. The whole site is runned by php file: prostredi.php

File : [Questionable .rar file link DELETED]

Thank you for any help... I can understand if you lock this topic :-) Sorry for my bad English...