Intensity in a point from two slits

[georg gill]

You have one slit which it is sent light through and in one point at a screen they measure the intensity with adding all the intensities from different rays who has different angels at the point. The E in the point is found from adding the E from all the rays and then you can find the intensity afterwards. Here is a picture of how you measure the E from all the rays. Here is an mage from my book:
http://www.flickr.com/photos/31101052@N06/5203631379/sizes/l/in/photostream/

I understand this formula 36.5 because the only variable in intensety is E and since 36.4 (from the text) has mesaured the new E you can see that the only difference between the new and the maximum field is what is after E₀ in36.4

But then it says that what is the I for a point when there are two slits and the I in a point in one lsit is affected from the waves from both. I don't see how this works. They use the formula for intensity for two rays found from a phasordiagram which in the end gives the formula 35.10 (shown in link below):
http://www.flickr.com/photos/31101052@N06/5203688911/sizes/l/in/photostream/

where I₀ is found from formula for intensity with E from two waves:
http://www.flickr.com/photos/31101052@N06/5203700111/sizes/l/in/photostream/

 

[Valerie Park]

I understand that the intensity is found by adding E fields but I don't understand how it works when you have two slits and the intensity in a point is affected from both. How does it work?