1. Use the centroid of a triangle
EXAMPLE 1
Use the centroid of a triangle
In RST, Q is the centroid and SQ = 8. Find QW and SW.
SOLUTION
SQ = 2 3 SW
Concurrency of Medians of a Triangle Theorem
8 = 2 3 SW
Substitute 8 for SQ.
Multiply each side by the reciprocal, . 3 2
12 = SW
Then QW =
SW – SQ =
12 – 8 = 4.
So, QW = 4 and SW = 12.

2. EXAMPLE 2
Standardized Test Practice
SOLUTION
Sketch FGH. Then use the Midpoint Formula to find the midpoint K of FH and sketch median GK . (See diagram on next slide.)
K( ) =
2 + 6 , 2
K(4, 3)
The centroid is two thirds of the distance from each vertex to the midpoint of the opposite side.

3. EXAMPLE 2
Standardized Test Practice
The distance from vertex G(4, 9) to K(4, 3) is 9 – 3 = 6 units. So, the centroid is (6) = 4 units down from G on GK . 2 3
The coordinates of the centroid P are (4, 9 – 4), or (4, 5).
The correct answer is B.

4. GUIDED PRACTICE
for Examples 1 and 2
There are three paths through a triangular park. Each path goes from the midpoint of one edge to the opposite corner. The paths meet at point P. 1.
If SC = 2100 feet, find PS and PC.
700 ft, 1400 ft
ANSWER

5. GUIDED PRACTICE
for Examples 1 and 2
There are three paths through a triangular park. Each path goes from the midpoint of one edge to the opposite corner. The paths meet at point P. 2.
If BT = 1000 feet, find TC and BC.
1000 ft, 2000 ft
ANSWER

6. GUIDED PRACTICE
for Examples 1 and 2
There are three paths through a triangular park. Each path goes from the midpoint of one edge to the opposite corner. The paths meet at point P. 3.
If PT = 800 feet, find PA and TA.
1600 ft, 2400 ft
ANSWER

7. EXAMPLE 3
Find the orthocenter
Find the orthocenter P in an acute, a right, and an obtuse triangle.
SOLUTION
Acute triangle
P is inside triangle.
Right triangle
P is on triangle.
Obtuse triangle
P is outside triangle.

8. EXAMPLE 4
Prove a property of isosceles triangles
Prove that the median to the base of an isosceles triangle is an altitude.
SOLUTION
GIVEN :
ABC is isosceles, with base AC . BD is the median to base AC .
PROVE :
BD is an altitude of ABC.

9. EXAMPLE 4
Prove a property of isosceles triangles
Proof : Legs AB and BC of isosceles ABC are congruent.
CD AD because BD is the median to AC . Also, BD BD . Therefore, ABD CBD by the SSS Congruence Postulate.
ADB CDB because corresponding parts of t triangles are Also, ADB and CDB are a linear pair. BD and AC intersect to form a linear pair of congruent angles, so BD AC and BD is an altitude of ABC.

10. GUIDED PRACTICE
for Examples 3 and 4 4.
Copy the triangle in Example 4 and find its orthocenter.
SOLUTION

11. GUIDED PRACTICE
for Examples 3 and 4 5.
WHAT IF?
In Example 4, suppose you wanted to show that median BD is also an angle bisector. How would your proof be different?
CBD
ABD
by SSS making
which leads to BD being an angle bisector.
ANSWER

12. GUIDED PRACTICE
for Examples 3 and 4 6.
Triangle PQR is an isosceles triangle and segment OQ is an altitude. What else do you know about OQ ? What are the coordinates of P?
OQ is also a perpendicular bisector, angle bisector, and median; (-h, 0).
ANSWER

13. Daily Homework Quiz
In Exercises 1–3, use the diagram.
G is the centroid of ABC.
1. If BG = 9, find BF.
ANSWER
13.5
2. If BD = 12, find AD.
ANSWER 12
3. If CD = 27, find GC.
ANSWER 18

14. Daily Homework Quiz
4. Find the centroid of ABC.
ANSWER
(1, 1)
5. Which type of triangle
has its orthocenter on the
triangle?
ANSWER
a right triangle