Question 906152
AC=AB+AB


AC=2*AB


Which implies that 2 copies of AB are needed to construct AC. Since AB + BC = AC, this must mean that BC = AB or AB = BC


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Think about it in reverse: say we know that AB = BC and AB + BC = AC


we can replace BC with AB to get 


AB + BC = AC


AB + AB = AC


AC = AB + AB


which is what we start with