Why Is the Angle Considered 90 Degrees?

[Joza]

http://www.e-xamit.ie/2005test/php/GetDocByID.php?id=562&prep=565&soln=562&tip=570&port=566


I can't figure out how it takes the angle dec to be 90 degrees? How?

 

[arildno]

Line segment ce is parallell to line segment ab. Line segment ad is perpendicular to ab at a, hence, ad is perpindicular to ec at e.