1. For A(–4, 8) and B(5, 8), find the midpoint of AB. ANSWER 1 2 , 8 2. For A(–3, 2) and B(4, –1), find the length of AB. ANSWER 58

3. For A(0, 4) and C(18, 4), find the length of AB, 3. For A(0, 4) and C(18, 4), find the length of AB, where B is a point the distance from A to C. 2 3 ANSWER 12

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.

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

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.

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

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

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

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.

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.

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 s 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.

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

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

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

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

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