Lesson: Parallel Lines in a Triangle Mathematics • First Year of Secondary School

Students will be able to

• recognize when parallel lines inside or outside triangles create similar triangles (sometimes called the side splitter theorem),
• use proportionality to find unknown lengths in triangles containing parallel lines,
• solve multistep problems involving triangles containing parallel lines,
• prove that a straight line is parallel to one side of a triangle due to having proportional parts (the inverse of the theorem).