Medians and Altitudes of Triangle Geometry Medians and Altitudes of Triangle CONFIDENTIAL

Warm up K 7 N Q P 9.1 40˚ 36˚ L J M CONFIDENTIAL

Medians and Altitudes of Triangle A median of a triangle is a segment whose endpoints are a vertex of the triangle and the midpoint of the opposite side. C median A B D Every triangle has three medians, and the median are concurrent, as shown in the next slide. CONFIDENTIAL

Construction Centroid of a Triangle B B X X Y Y A C A C Z Z Next page : CONFIDENTIAL

B X P Y C A Z CONFIDENTIAL

The point of concurrency of the medians of a triangle is the centroid of the triangle. The centroid is always inside the triangle. The centroid is also called the center of gravity because it is the point where a triangular region will balance. CONFIDENTIAL

Theorem 3.1 Centroid Theorem The centroid of a triangle is located 2 of the distance from 3 each vertex to the midpoint of the opposite side. B X Y P A C Z CONFIDENTIAL

Using the Centroid to Find Segment Lengths In ∆ABC, AF = 9, and GE = 2.4. Find each length. B A) AG E F G Centroid Thm. Substitute 9 for AF Simplify. A C D Next page : CONFIDENTIAL

Subtract 2CE from both sides. B) CE E F G Centroid Thm. Seg. Add. Post. Substitute 2 CE for CG. 3 Subtract 2CE from both sides. Substitute 2.4 for GE. Multiply both sides by 3. A C D CONFIDENTIAL

Now you try! 1) In ∆JKL, ZW =7,and LX = 8.1. Find each length. KW LZ X CONFIDENTIAL

Problem-Solving Application The diagram shows the plan for a triangular piece of a mobile. Where should the sculptor attach the support so that the triangle is balanced? y Q(0, 8) 8 6 1) Understand the Problem R(6, 4) 4 The answer will be the coordinates of the centroid of ∆PQR. The important information is the location of the vertices, P(3,0),Q(0,8), and R(6,4). 2 P(3, 0) x 2 4 6 8 Next page : CONFIDENTIAL

2) Make a Plan y Q(0, 8) The centroid of the triangle is the point of intersection of the three medians. So write the equations for two medians and find their point of intersection. 8 6 R(6, 4) 4 2 P(3, 0) x 2 4 6 8 CONFIDENTIAL

y Q(0, 8) 8 6 R(6, 4) 4 2 P(3, 0) x 2 4 6 8 CONFIDENTIAL

Now you try! 2) Find the average of the x-coordinates and the average of the y-coordinates of the vertices of ∆PQR. Make a conjecture about the centroid of a triangle. y Q(0, 8) 8 6 R(6, 4) 4 2 P(3, 0) x 2 4 6 8 CONFIDENTIAL

In ∆QRS, altitude QY is inside the triangle, but RX and SZ are not. An altitude of a triangle is a perpendicular segment from a vertex to the line containing the opposite side. Every triangle has three altitudes. An altitude can be inside, outside, or on the triangle. R In ∆QRS, altitude QY is inside the triangle, but RX and SZ are not. Notice that the lines containing the altitudes are concurrent at P. This point of concurrency is the orthocenter of the triangle. Y Q X P Z S CONFIDENTIAL

Finding the Orthocenter Find the orthocenter of ∆JKL with vertices J(-4,2), K(-2,6), and L(2,2). Step 1 Graph the triangle. Step 2 Find an equation of the line containing the altitude from K to JL. Since JL is horizontal, the altitude is vertical. The line containing it must pass through K(-2,6), so the equation of the line is x = -2. y X = -2 K 7 (-2, 4) J L x -4 2 Y = x + 6 Next page : CONFIDENTIAL

Step 3 Find an equation of the line containing the altitude from J to KL. y X = -2 The slope of a line perpendicular to KL is 1. This line must pass through J(-4, 2). K 7 (-2, 4) J L x -4 2 Y = x + 6 Next page : CONFIDENTIAL

Step 4 Solve the system to find the coordinates of the orthocenter. X = -2 Step 4 Solve the system to find the coordinates of the orthocenter. x = -2 y = x + 2 y = -2 + 6 = 4 The coordinates of the orthocenter are (-2,4). K 7 (-2, 4) J L x -4 2 Y = x + 6 CONFIDENTIAL

Now you try! 3) Show that the altitude to JK passes through the orthocenter of ∆JKL. y X = -2 K 7 (-2, 4) J L x -4 2 Y = x + 6 CONFIDENTIAL

Now some problems for you to practice ! CONFIDENTIAL

VX = 205, and RW = 104. Find each length. Assessment VX = 205, and RW = 104. Find each length. 1) VW 2) WX 3) RY 4) WY T Y X W V R CONFIDENTIAL

5) The diagram shows a plan for a piece of a mobile 5) The diagram shows a plan for a piece of a mobile. A chain will hang from the centroid of the triangle. At what coordinates should the artist attach the chain? y B(7, 4) 4 2 A(0, 2) x 2 C(5, 0) 8 CONFIDENTIAL

Find the orthocenter of a triangle with the given vertices. K(2, -2), L(4, 6), M(8, -2) U(-4, -9), V(-4, 6),W(5, -3) P(-5, 8), Q(4, 5), R(-2, 5) C(-1, -3), D(-1, 2), E(9, 2) CONFIDENTIAL

Medians and Altitudes of Triangle Let’s review Medians and Altitudes of Triangle A median of a triangle is a segment whose endpoints are a vertex of the triangle and the midpoint of the opposite side. C median A B D Every triangle has three medians, and the median are concurrent, as shown in the construction below. CONFIDENTIAL

Construction Centroid of a Triangle B B X X Y Y A C A C Z Z Next page : CONFIDENTIAL

B X P Y C A Z CONFIDENTIAL

The point of concurrency of the medians of a triangle is the centroid of the triangle. The centroid is always inside the triangle. The centroid is also called the center of gravity because it is the point where a triangular region will balance. CONFIDENTIAL

Theorem 3.1 Centroid Theorem The centroid of a triangle is located 2/3 of the distance from each vertex to the midpoint of the opposite side. B X Y P A C Z CONFIDENTIAL

Using the Centroid to Find Segment Lengths In ∆ABC, AF = 9, and GE = 2.4. Find each length. B A) AG E F G Centroid Thm. Substitute 9 for AF Simplify. A C D Next page : CONFIDENTIAL

Subtract 2/3 CE from both sides. B) CE E G Centroid Thm. Seg. Add. Post. Substitute 2/3 CE for CG. Subtract 2/3 CE from both sides. Substitute 2.4 for GE. Multiply both sides by 3. A D CONFIDENTIAL

Problem-Solving Application The diagram shows the plan for a triangular piece of a mobile. Where should the sculptor attach the support so that the triangle is balanced? y Q(0, 8) 8 6 1) Understand the Problem R(6, 4) 4 The answer will be the coordinates of the centroid of ∆PQR. The important information is the location of the vertices, P(3,0),Q(0,8), and R(6,4). 2 P(3, 0) x 2 4 6 8 Next page : CONFIDENTIAL

2) Make a Plan y Q(0, 8) The centroid of the triangle is the point of intersection of the three medians. So write the equations for two medians and find their point of intersection. 8 6 R(6, 4) 4 2 P(3, 0) x 2 4 6 8 CONFIDENTIAL

y Q(0, 8) 8 6 R(6, 4) 4 2 P(3, 0) x 2 4 6 8 CONFIDENTIAL

In ∆QRS, altitude QY is inside the triangle, but RX and SZ are not. An altitude of a triangle is a perpendicular segment from a vertex to the line containing the opposite side. Every triangle has three altitudes. An altitude can be inside, outside, or on the triangle. R In ∆QRS, altitude QY is inside the triangle, but RX and SZ are not. Notice that the lines containing the altitudes are concurrent at P. This point of concurrency is the orthocenter of the triangle. Y Q X P Z S CONFIDENTIAL

Finding the Orthocenter Find the orthocenter of ∆JKL with vertices J(-4,2), K(-2,6), and L(2,2). Step 1 Graph the triangle. Step 2 Find an equation of the line containing the altitude from K to JL. Since JL is horizontal, the altitude is vertical. The line containing it must pass through K(-2,6), so the equation of the line is x = -2. y X = -2 K 7 (-2, 4) J L x -4 2 Y = x + 6 Next page : CONFIDENTIAL

Step 3 Find an equation of the line containing the altitude from J to KL. y X = -2 The slope of a line perpendicular to KL is 1. This line must pass through J(-4, 2). K 7 (-2, 4) J L x -4 2 Y = x + 6 Next page : CONFIDENTIAL

Step 4 Solve the system to find the coordinates of the orthocenter. X = -2 Step 4 Solve the system to find the coordinates of the orthocenter. x = -2 y = x + 2 y = -2 + 6 = 4 The coordinates of the orthocenter are (-2,4). K 7 (-2, 4) J L x -4 2 Y = x + 6 CONFIDENTIAL

You did a great job today! CONFIDENTIAL