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# Why is the 0 angle axiom on Straight line theorem equals to -`the angle` ?

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1. Just researching maths a little , can't seem to get my mind across this ,for example ,
If there are 3 distinct points , A , B , C and B lies between A C then the angle BAC being 0
then to prove that the ABC=180
Where if you would try to construct a triangle , according to SAS congruence
AB=AB , AC=AC
Where apperantly , the <BAC = -<BAC
I need help on understanding , why there's a `minus` sign for the congruent angle ?
2. (Original post by OzyCozy)
Just researching maths a little , can't seem to get my mind across this ,for example ,
If there are 3 distinct points , A , B , C and B lies between A C then the angle BAC being 0
then to prove that the ABC=180
Where if you would try to construct a triangle , according to SAS congruence
AB=AB , AC=AC
Where apperantly , the <BAC = -<BAC
I need help on understanding , why there's a `minus` sign for the congruent angle ?
Directed angles.

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