A Level Maths and Calculus
https://www.youtube.com/user/TheIntegralCALC
https://www.cambridge.org/gb/education/subject/mathematics/a-level-further-mathematics-ocr-a/a-level-further-mathematics-ocr-a-pure-core-student-book-1-as-year-1?isbn=9781316644386&format=PB
https://www.youtube.com/playlist?list=PL0F87042C76183259
https://www.youtube.com/playlist?list=PL0F87042C76183259
A Proof and a Generalization of the the Theorem can be found here THEOREMOFVANAUBEL GENERALIZATIONOFVANAUBELSTHEOREM
Detailshttp://www.acoustics.salford.ac.uk/feschools/waves/diffract3.php
http://www.zmescience.com/science/math/10-beautiful-visual-mathematical-proofs-elegance-and-simplicity/