8. Hình thang ABCD (AB // CD) có \(\widehat{A}-\widehat{D}=\)200 , \(\widehat{B}=2\widehat{C}\)Tính các góc của hình thang.
Ta có \(\widehat{A}-\widehat{D}=\)200; \(\widehat{A}+\widehat{D= }\) 1800
Từ \(\widehat{A}-\widehat{D}=\)200
=> \(\widehat{A}\)= 200 +\(\widehat{D}\)
Nên \(\widehat{A}+\widehat{D}=\) 200 + \(\widehat{D}\) +\(\widehat{D}\)=200 +2 \(\widehat{D}\) =1800
=> 2\(\widehat{D}\)=1600 => \(\widehat{D}\)= 800
Thay \(\widehat{D}\)= 800 vào \(\widehat{A}\)= 200 +\(\widehat{D}\) ta được \(\widehat{A}\)=200 + 800 = 1000
Lại có \(\widehat{B}=2\widehat{C}\) ; \(\widehat{B}+\widehat{C}=\)1800
nên \(2\widehat{C}+\widehat{C}=\)1800
hay \(3\widehat{C}=\)1800 => \(\widehat{C}=\)600
Do đó \(\widehat{B}=2\widehat{C}\)= 2.600
=> \(\widehat{B}\)=1200