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Prove suppose A,B and C lie on a line L, with B between A and C, and that D is not on the line L. Prove that angle DBA + angle DBC = 180

this question is obvious. But because its obvious its hard to prove.

Say that X = angle DBA and Y = angle DBC

therefor cos(X + Y) = -1 ... ive gotten this far, i just dont know how to obtain anymore information

could i do cos(X) = u v/||u||||v|| and cos(Y) = v w/||v||||w||

where v is the vector shared with the two angles..

what do i do

thanks

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# Homework Help: Geometry help

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