Ta có:
`hat{B} = 2 hat{C} = 2 . 100^o = 200^o`
Tứ giác `ABCD` có `hat{A} + hat{B} + hat{C} + hat{D} = 360^o`
`=> hat{A} + 200^o + 100^o + hat{D} = 360^o`
`=> hat{A} + hat{D} = 60^o`
`=> hat{A} = (60^o + 20^o)/2 = 40^o`
`hat{D} = 40^o - 20^o = 20^o`
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