5.3: Medians and Altitudes in Triangles
Learning Objectives
 Define median and find the properties of the centroid.
 Apply medians to the coordinate plane.
 Construct the altitude of a triangle.
Review Queue
 Find the midpoint between (9, 1) and (1, 15).
 Find the slope between (9, 1) and (1, 15). Then find the equation of the line.
 Find the equation of the line that is perpendicular to the line from #2 through (6, 2).
Know What? Triangles are frequently used in art. Your art teacher assigns an art project involving triangles. You decide to make a series of hanging triangles of all different sizes from one long piece of wire. Where should you hang the triangles from so that they balance horizontally?
Medians
Median: The line segment that joins a vertex and the midpoint of the opposite side (of a triangle).
is the median from to the midpoint of .
Example 1: Find the other two medians of .
Solution: Find the midpoints of sides and , using a ruler. Be sure to always include the appropriate tick marks for the midpoints.
Centroid: The point of intersection for the medians of a triangle.
Investigation 55: Properties of the Centroid
Tools Needed: pencil, paper, ruler, compass
1. Construct a scalene triangle with sides of length 6 cm, 10 cm, and 12 cm (Investigation 42). Use the ruler to measure each side and mark the midpoint.
2. Draw in the medians and mark the centroid.
Measure the length of each median. Then, measure the length from each vertex to the centroid and from the centroid to the midpoint. Do you notice anything?
3. Cut out the triangle. Place the centroid on either the tip of the pencil or the pointer of the compass. What happens?
The properties discovered are summarized below.
Median Theorem: The medians of a triangle intersect at a point that is twothirds of the distance from the vertices to the midpoint of the opposite side. The centroid is the “balancing point” of a triangle.
If is the centroid, then:
Example 2: , and are midpoints of the sides of .
a) If , find and .
b) If , find and .
Solution:
a) . .
b)
. is a third of 21, .
Example 3: Algebra Connection is the centroid of and . Find and .
Solution:
Altitudes
The last line segment within a triangle is an altitude. It is also called the height of a triangle.
Altitude: A line segment from a vertex and perpendicular to the opposite side. The red lines below are all altitudes.
When a triangle is a right triangle, the altitude, or height, is the leg. If the triangle is obtuse, then the altitude will be outside of the triangle.
Investigation 56: Constructing an Altitude for an Obtuse Triangle
Tools Needed: pencil, paper, compass, ruler
1. Draw an obtuse triangle. Label it , like the picture to the right. Extend side , beyond point .
2. Using Investigation 32, construct a perpendicular line to , through .
The altitude does not have to extend past side , as it does in the picture. Technically the height is only the vertical distance from the highest vertex to the opposite side.
If you are constructing an altitude for an acute triangle, you may skip Step 1 of this investigation.
Know What? Revisited The point that you should put the wire through is the centroid. That way, each triangle will balance.
Review Questions
 Questions 14 use Investigation 55.
 Questions 56 use Investigation 32 and 56.
 Questions 718 are similar to Examples 2 and 3.
 Questions 1926 use review to discover something new.
 Questions 2734 use the definitions of perpendicular bisector, angle bisector, median and altitude.
 Question 35 is similar to the proofs in the previous section.
Construction Construct the centroid for the following triangles by tracing each triangle onto a piece of paper and using Investigation 55.
 Is the centroid always going to be inside of the triangle? Why?
Construction Construct the altitude from the top vertex for the following triangles. Trace each triangle onto a piece of paper and using Investigations 32 and 56.
For questions 711, , and are the midpoints of each side and is the centroid. Find the following lengths.
 If , find and
 If , find and
 If , find and
 If , find and
 If and , find and .
For questions 1218, and are the midpoints of sides and .
 What is point ?
 If , find .
 If , find .
 If , find .
 If , find .
 If , find .
 If and , find and .
Multistep Problem Find the equation of a median in the plane.
 Plot and
 Find the midpoint of . Label it .
 Find the slope of .
 Find the equation of .
 Plot
 Find the midpoint of . Label it .
 Find the slope of .
 Find the equation of .
Determine if the following statements are true or false.
 The perpendicular bisector goes through the midpoint of a line segment.
 The perpendicular bisector goes through a vertex.
 The angle bisector passes through the midpoint.
 The median bisects the side it intersects.
 The angle bisectors intersect at one point.
 The altitude of an obtuse triangle is inside a triangle.
 The centroid is the balancing point of a triangle.
 A median and a perpendicular bisector intersect at the midpoint of the side they intersect.
Fill in the blanks in the proof below.
 Given: Isoscles with legs and and Prove:
Statement  Reason 

1. Isosceles with legs and and 

2.  
3.  Definition of perpendicular lines 
4.  
5.  Reflexive PoC 
6.  
7. 