Đáp án+Giải thích các bước giải:
`\hat{A}+\hat{D}=\hat{B}+\hat{C}`
`⇒2\hat{D}+\hat{D}=\hat{C}+20^o +\hat{C}`
`⇒3\hat{D}=2\hat{C}+20^o`
`\hat{A}+\hat{B}+\hat{C}+\hat{D}=180^o`
`⇒3\hat{D}+2\hat{C}+20^o=180^o`
Mà `3\hat{D}=2\hat{C}+20^o(cmt)`
`⇒6\hat{D}=180^o`
`⇒\hat{D}=30^o`
`⇒\hat{A}=2\hat{D}=2.30^o=60^o`
`3\hat{D}=2\hat{C}+20^o`
`⇒\hat{C}=\frac{3.30^o -20^o}{2}=35^o`
`⇒\hat{B}=\hat{C}+20^o=35^o +20^o=55^o`
`⇒\hat{A}+2\hat{B}-\hat{C}-3\hat{D}`
`=60^o +2.55^o -35^o -3.30^o`
`=45^o`
