Hello. Here I understand why angle 'AED' is a supplement for angle 'DEC', but why is 'ABC'? Thanks.
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Hello, norifurippu! Here I understand why $\displaystyle \angle AED$ is a supplement for $\displaystyle \angle DEC$, but why is $\displaystyle \angle ABC$ ? $\displaystyle BCED$ 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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