Given figure has two triangle, TRS & CAB
where, RS is parallel to AB
RT is parallel to AC
we can proof AC/RT = AB/RS by applying similar triangle properties by AAA method
Since,
AB is parallel to RS and TS is the transversal line,
so,

AC is parallel to RTand TS is transversal line,
so,

The sum of all angles in a triangle is 180 degree,
then In triangle ABC.

So, we have now,

so, by Angle Angle Angle methodology, traingle ABC and RST are similarl.

In similarl triangle, The ratio of corresponding sides is the same,
so,

thus,

hence proof,