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What if you were told that J , K , and L were the midpoints of the sides of \triangle FGH and that M was the centroid of \triangle FGH ? Given the length of JK , how could you find the lengths of JM and KM ? After completing this Concept, you'll be able to use the Median Theorem to solve problems like this one.

Watch This

First watch this video.

James Sousa: Medians of a Triangle

Now watch this video.

James Sousa: Using the Properties of Medians to Solve for Unknown Values

Guidance

In a triangle, the line segment that joins a vertex and the midpoint of the opposite side is called a median .

\overline{LO} is the median from L to the midpoint of \overline{NM} .

If you draw all three medians they will intersect at one point called the centroid .

The centroid is the “balancing point” of a triangle. This means that if you were to cut out the triangle, the centroid is its center of gravity so you could balance it there.

The Median Theorem states that the medians of a triangle intersect at a point called the centroid that is two-thirds of the distance from the vertices to the midpoint of the opposite sides.

So if G is the centroid, then:

AG &= \frac{2}{3} AD, \ CG = \frac{2}{3} CF, \ EG = \frac{2}{3} BE\\DG &= \frac{1}{3} AD, \ FG = \frac{1}{3} CF, \ BG = \frac{1}{3} BE\\\text{And by substitution}: \quad DG &= \frac{1}{2} AG, \ FG = \frac{1}{2} CG, \ BG = \frac{1}{2} EG

Example A

I, \ K , and M are midpoints of the sides of \triangle HJL .

a) If JM = 18 , find JN and NM .

b) If HN = 14 , find NK and HK .

To solve, use the Median Theorem.

a) JN = \frac{2}{3} \cdot 18 = 12 . NM = JM - JN = 18 - 12 . NM = 6.

b) 14 = \frac{2}{3} \cdot HK

14 \cdot \frac{3}{2} = HK = 21 . NK is a third of 21, NK = 7 .

Example B

H is the centroid of \triangle ABC and DC = 5y - 16 . Find x and y .

To solve, use the Median Theorem. Set up and solve equations.

\frac{1}{2} BH= HF & \longrightarrow BH = 2HF && HC = \frac{2}{3} DC \longrightarrow \frac{3}{2} HC = DC\\3x + 6 &= 2(2x - 1) && \quad \ \frac{3}{2} (2y + 8) = 5y - 16\\3x + 6 &= 4x - 2 && \qquad \ 3y + 12 = 5y - 16\\8 &= x && \qquad \ \ \qquad 28 = 2y \longrightarrow 14 = y

Example C

B, \ D , and F are the midpoints of each side and G is the centroid. If BG = 5 , find GE and BE

Use the Median Theorem.

BG&=\frac{1}{3}BE \\ 5 &=\frac{1}{3}BE \\ BE &=15\\ .

Therefore, GE=10 .

CK-12 Medians

Guided Practice

1. B, \ D , and F are the midpoints of each side and G is the centroid. If CG = 16 , find GF and CF

2. True or false: The median bisects the side it intersects.

3. N and M are the midpoints of sides \overline{XY} and \overline{ZY} .

a. What is point C ?

b. If XN = 5 , find XY .

c. If ZN = 6x + 15 and ZC = 38 , find x and ZN .

Answers

1. Use the Median Theorem.

CG&=\frac{2}{3}CF \\ 16 &=\frac{2}{3}CF \\ CF &=24\\ .

Therefore, GF=8

2. This statement is true. By definition, a median intersects a side of a triangle at its midpoint. Midpoints divide segments into two equal parts.

3. Use the Median Theorem.

a. C is the centroid.

b. XN=\frac{1}{2}XY , so XY=10 .

c.

 ZC&=\frac{2}{3}ZN\\38&=\frac{2}{3}(6x+15)\\57&=6x+15\\42&=6x\\x&=7 .

Substitute 7 for x to find that ZN=57 .

Practice

For questions 1-4, B, \ D , and F are the midpoints of each side and G is the centroid. Find the following lengths.

  1. If CG = 16 , find GF and CF
  2. If AD = 30 , find AG and GD
  3. If GF = x , find GC and CF
  4. If AG = 9x and GD = 5x - 1 , find x and AD .

Multistep Problem Find the equation of a median in the x-y plane.

  1. Plot \triangle ABC: \ A(-6, 4), \ B(-2, 4) and C(6, -4)
  2. Find the midpoint of \overline{AC} . Label it D .
  3. Find the slope of \overline{BD} .
  4. Find the equation of \overline{BD} .
  5. Plot \triangle DEF: \ D(-1, 5), \ E(0, -1), \ F(6, 3)
  6. Find the midpoint of \overline{EF} . Label it G .
  7. Find the slope of \overline{DG} .
  8. Find the equation of \overline{DG} .

Determine whether the following statement is true or false.

  1. The centroid is the balancing point of a triangle.

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