5.4 Use medians and altitudes

1. 5.4 Use medians and altitudes Objective: You will use medians and altitudes of triangles.

2. Median of a triangle: • A segment from a vertex to the midpoint of the opposite side. • Three medians of a triangle are concurrent. The point of concurrency, called the centroid, is inside the triangle.

3. THEOREM:Concurrency of Medians of a Triangle • The medians of a triangle intersect at a point that is 2/3 the distance from each vertex to the midpoint of the opposite side. • AP=2/3AE

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

5. Sketch FGH. Then use the Midpoint Formula to find the midpoint Kof FHand sketch median GK. 2 + 6 , 5 + 1 K( ) = 2 2 EXAMPLE 2 Standardized Test Practice SOLUTION K(4, 3) The centroid is two thirds of the distance from each vertex to the midpoint of the opposite side.

6. 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. The correct answer is B. 2 3 EXAMPLE 2 Standardized Test Practice The coordinates of the centroid Pare (4, 9 – 4), or (4, 5).

7. 1. If SC = 2100 feet, findPS andPC. ANSWER 700 ft, 1400 ft for Examples 1 and 2 GUIDED PRACTICE 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.

8. 2. If BT = 1000 feet, find TC andBC. ANSWER 1000 ft, 2000 ft for Examples 1 and 2 GUIDED PRACTICE 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.

9. 3. If PT = 800 feet, findPA andTA. ANSWER 1600 ft, 2400 ft for Examples 1 and 2 GUIDED PRACTICE 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.

10. Altitude of a triangle: • The perpendicular segment from a vertex to the opposite side or to the line that contains the opposite side.

12. THEOREM:Concurrency of Altitudes of a Triangle. • The lines containing the altitudes of a triangle are congruent. • The lines containing AF, BE, and CD meet at G.

13. Orthocenter • The point at which the lines containing the three altitudes of a triangle intersect.

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

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

16. CD ADbecause BDis the median to AC. Also, BDBD. Therefore, ABD CBDby the SSS Congruence Postulate. ADB CDB because corresponding parts of s are . Also, ADB and CDBare a linear pair. BDand ACintersect to form a linear pair of congruent angles, so BD  ACand BDis an altitude of ABC. EXAMPLE 4 Prove a property of isosceles triangles Proof : Legs ABand BCof isosceles ABCare congruent.

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

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

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