Here I understand why angle 'AED' is a supplement for angle 'DEC', but why is 'ABC'?
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Here I understand why is a supplement for , but why is ? is a cyclic quadrilateral (inscribed in a circle). . . Hence, its opposite angles are supplementary.
Thank you very much Soroban!
you could just reason it as m<AED is a supplement to m<DEC, m<ABC = m<AED, thus they have the same measure, so m<ABC is also a supplement of m<DEC.
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