円に内接する四角形の対角をたすと180°になります
よって、∠ADC＝180°－x
したがって、∠EDC＝x
また、∠DCE＝21°＋x
ゆえに、△DCEより
21°＋x＋x＋33°＝180°
2x＝126°
x＝63°
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