chú thích : tg : tứ giác
Xét tg `ABCD` có :
\(\widehat{A}+\widehat{B}+\widehat{C}+\widehat{D}=360^0\)(tổng 4 góc của 1 tg)
`=>`\(145^0+99^0+\widehat{C}+\widehat{D}=360^0\)
Vì \(\widehat{C}=3\widehat{D}\)
`=>`\(\widehat{D}+3\widehat{D}=116^0\)
`=>`\(\widehat{D}=29^0\)
`=>`\(\widehat{C}=3\cdot29^0=87^0\)
Xét tứ giác ABCD có \(\widehat{A}+\widehat{B}+\widehat{C}+\widehat{D}=360^0\)
=>\(\widehat{C}+\widehat{D}=360^0-145^0-99^0=116^0\)
=>\(3\cdot\widehat{D}+\widehat{D}=116^0\)
=>\(4\cdot\widehat{D}=116^0\)
=>\(\widehat{D}=\dfrac{116^0}{4}=29^0\)
\(\widehat{C}=3\cdot29^0=87^0\)
