1:
Ax//Ot
=>\(\widehat{O_1}=\widehat{xAO}\)(hai góc so le trong)
=>\(\widehat{O_1}=30^0\)
Ot//By
=>\(\widehat{O_2}+\widehat{yBO}=180^0\)(hai góc trong cùng phía)
=>\(\widehat{O_2}=180^0-150^0=30^0\)
\(\widehat{AOB}=\widehat{O_1}+\widehat{O_2}=30^0+30^0=60^0\)
2:
a//b
=>\(\widehat{A_1}=\widehat{B_1}\)(hai góc đồng vị)
mà \(\widehat{A_1}=60^0\)
nên \(\widehat{B_1}=60^0\)
\(\widehat{B_1}+\widehat{B_2}=180^0\)(hai góc kề bù)
=>\(\widehat{B_2}+60^0=180^0\)
=>\(\widehat{B_2}=120^0\)
3:
Kẻ Bz//Ax
Bz//Ax
=>\(\widehat{xAB}+\widehat{zBA}=180^0\)(hai góc trong cùng phía)
=>\(\widehat{zBA}=180^0-140^0=40^0\)
\(\widehat{zBA}+\widehat{zBC}=\widehat{ABC}\)
=>\(\widehat{zBC}=70^0-40^0=30^0\)
\(\widehat{zBC}+\widehat{yCB}=180^0\)
mà hai góc này là hai góc ở vị trí trong cùng phía
nên Bz//Cy
mà Bz//Ax
nên Ax//Cy
