Đáp án+Giải thích các bước giải:
`1)`
Ta có: `\hat{B}=2\hat{A}=2x`
`\hat{C}=3\hat{A}=3x`
`\hat{D}=2\hat{B}=2.2\hat{A}=4x`
Tứ giác `ABCD` có: `\hat{A}+\hat{B}+\hat{C}+hat{D}=360^o`
hay `x + 2x + 3x + 4x = 360^o`
`=> 10x = 360^o`
`=> x =360^o:10=36^o`
`=> B= 2x=2.36^o=72^o`
`=> \hat{C}=3x=3.36^o=108^o`
`=> \hat{D}=4x=4.36^o=144^o`
`2)`
`a)` Ta có: `AB bot AD(g``t)`
`AB bot BC(g``t)`
`=> AD //// BC`
`b)`Tứ giác `ABCD` có: `\hat{A}+\hat{B}+\hat{C}+hat{D}=360^o`
hay `90^o``+90^o``+\hat{C}+hat{D}=360^o`
`=> \hat{C}+hat{D}=360^o-90^o-90^o=180^o`
mà `\hat{C}=3\hat{D}(g``t)`
`=> 3\hat{D}+hat{D}=180^o`
`=> 4\hat{D}=180^o`
`=> \hat{D}=180^o:4=45^o`
`=> \hat{C}= 3.45^o=135^o`
