Trong tứ giác `ABCD` có :
\(\widehat{A}+\widehat{B}+\widehat{C}+\widehat{D}=360^0\)
hay `80^0 + 50^0 + 100^0`\(+\widehat{D}=360^0\)
`=>`\(\widehat{D}=360-\left(80+50+100\right)\)
`=>`\(\widehat{D}=360-\left(80+50+100\right)\)
`=>`\(\widehat{D}=130^0\)