91Ó°ÊÓ

The measurement of angles that are positioned on alternate interior side is equal when the lines related to the angles are having a parallel relation.

Draw a line S that is having a parallel relation with existing parallel lines.

According to the obtained figure the lines XR and US are having a parallel relation and a separate line RS is travelling through them so according to the method the measurement of $∠1$and $∠SRX$will be the same.

$∠1=∠SRX=40°$

According to the obtained figure the lines YT and US are having a parallel relation and a separate line ST is travelling through them so according to the method the measurement of $∠2$and $∠STY$will be the same.

$∠2=∠STY=70°$

According to the obtained figure $∠RST$is the addition of $∠1$and$∠2$ so determine the measurement of $∠RST$.

$∠RST=∠1+∠2$

$=40°+70°$

$=110°$