MidSegments in Triangles - MathBitsNotebook (Geo) (2024)

MidSegments in Triangles - MathBitsNotebook (Geo) (1)

Mid-Segments in Triangles
MathBitsNotebook.com

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MidSegments in Triangles - MathBitsNotebook (Geo) (2)


MidSegments in Triangles - MathBitsNotebook (Geo) (3)

The mid-segment of a triangle (also called a midline) is a segment joining the midpoints of two sides of a triangle.


MidSegments in Triangles - MathBitsNotebook (Geo) (4)

"Mid-Segment Theorem": The mid-segment of a triangle, which joins the midpoints of two sides of a triangle, is parallel to the third side of the triangle and half the length of that third side of the triangle.

MidSegments in Triangles - MathBitsNotebook (Geo) (5)

MidSegments in Triangles - MathBitsNotebook (Geo) (6)

Examples:

1. MidSegments in Triangles - MathBitsNotebook (Geo) (7)

Given M, N midpoints.
MN = 12
Find DF.

ANSWER:
MidSegments in Triangles - MathBitsNotebook (Geo) (8)

2. MidSegments in Triangles - MathBitsNotebook (Geo) (9)

Given D, E midpoints.
DE = 3x - 5
AB = 26
Find x.

ANSWER:
MidSegments in Triangles - MathBitsNotebook (Geo) (10)

3. MidSegments in Triangles - MathBitsNotebook (Geo) (11)

Given right ΔRST.
G, N, J midpoints.
ST = 6; RS = 8
Find perimeter of ΔGNJ.

ANSWER:
MidSegments in Triangles - MathBitsNotebook (Geo) (12)

Proof of Mid-Segment Theorem - Using Coordinate Geometry


For this proof, the diagram has been positioned in the first quadrant with one side on the x-axis to keep the algebraic computations as simple as possible, without losing the general positioning of the triangle. Be aware that other positionings are also possible.

MidSegments in Triangles - MathBitsNotebook (Geo) (13)

MidSegments in Triangles - MathBitsNotebook (Geo) (14)

Coordinate Geometry formulas needed for this proof:

Midpoint Formula: MidSegments in Triangles - MathBitsNotebook (Geo) (15)

Distance Formula: MidSegments in Triangles - MathBitsNotebook (Geo) (16)

Proof:
MidSegments in Triangles - MathBitsNotebook (Geo) (17)

Proof of Mid-Segment Theorem - Using Similar Triangles


For this proof, we will prove ΔMFN is similar ΔDFE, by SAS for similar triangles, to obtain corresponding angles for parallel lines and establish a pair of proportional sides.

MidSegments in Triangles - MathBitsNotebook (Geo) (18)

MidSegments in Triangles - MathBitsNotebook (Geo) (19)

Statements

Reasons

1. MidSegments in Triangles - MathBitsNotebook (Geo) (20)

1. Given

2. MidSegments in Triangles - MathBitsNotebook (Geo) (21)

2. A mid-segment joins the midpoints of two sides of a triangle.

3. MidSegments in Triangles - MathBitsNotebook (Geo) (22)

3. Midpoint of a segment divides a segment into 2 congruent segments.

4. DM = MF; FN = NE

4. Congruent segments are segments of = length.

5. DM + MF = DF; FN + NE = FE

5. Segment Addition Postulate (or Whole Quantity)

6. MF + MF = DF; FN + FN = FE

6. Substitution

7. 2MF = DF; 2FN = FE

7. Addition (or Combine Like Terms)

8. MidSegments in Triangles - MathBitsNotebook (Geo) (23); MidSegments in Triangles - MathBitsNotebook (Geo) (24)

8. Multiplication (or Division) property of equality.
[This step establishes the ratio of similitude between the two triangles.]

9. MidSegments in Triangles - MathBitsNotebook (Geo) (25)

9. Reflexive Property (or Identity Property)

10. MidSegments in Triangles - MathBitsNotebook (Geo) (26)

10. SAS for Similar Triangles: If an ∠ of one Δ is congruent to the corresponding ∠ of another Δ and the lengths of the sides including these ∠s are in proportion, the Δs are similar.

11. MidSegments in Triangles - MathBitsNotebook (Geo) (27)

11. Corresponding angles in similar triangles are congruent.

12. MidSegments in Triangles - MathBitsNotebook (Geo) (28)

12. If 2 lines are cut by a transversal such that the corresponding angles are congruent, the lines are parallel.

13. MidSegments in Triangles - MathBitsNotebook (Geo) (29)

13. Corresponding sides of similar triangles are in proportion. QED.

Proof of Mid-Segment Theorem - Using Parallelogram


For this proof, we will utilize an auxiliary line, congruent triangles and the properties of a parallelogram.

MidSegments in Triangles - MathBitsNotebook (Geo) (30)

MidSegments in Triangles - MathBitsNotebook (Geo) (31)

Statements

Reasons

1. MidSegments in Triangles - MathBitsNotebook (Geo) (32)

1. Given

2. MidSegments in Triangles - MathBitsNotebook (Geo) (33)

2. A mid-segment joins the midpoints of two sides of a triangle.

3. Through E draw line parallel to MidSegments in Triangles - MathBitsNotebook (Geo) (34). Extend MidSegments in Triangles - MathBitsNotebook (Geo) (35) to intersect at M1.

3. Through a point not on a line, only one line can be drawn parallel to the given line. Parallel Postulate.

4. MidSegments in Triangles - MathBitsNotebook (Geo) (36)

4. Midpoint of a segment divides a segment into 2 congruent segments.

5. DFE MidSegments in Triangles - MathBitsNotebook (Geo) (37)FEM1

5. If 2 parallel lines are cut by a transversal, the alternate interior angles are congruent.

6.FNM MidSegments in Triangles - MathBitsNotebook (Geo) (38)∠M1NE

6. Vertical angles are congruent.

7. ΔFNM MidSegments in Triangles - MathBitsNotebook (Geo) (39)ΔM1NE

7. ASA - If 2∠s and the included side of one Δ are congruent to the corresponding parts of another Δ, the Δs are congruent.

8. MidSegments in Triangles - MathBitsNotebook (Geo) (40)

8. CPCTC - corresponding parts of congruent triangles are congruent.

9. MidSegments in Triangles - MathBitsNotebook (Geo) (41)

9. Substitution (or Transitive property)

10. DMM1E is a parallelogram

10. A quadrilateral with one pair of sides both || and congruent is a parallelogram.

11. MidSegments in Triangles - MathBitsNotebook (Geo) (42)

11. A parallelogram is a quad. with 2 pair of opposite sides parallel.

12. MidSegments in Triangles - MathBitsNotebook (Geo) (43)

12. Opposite sides of a parallelogram are congruent.

13. MidSegments in Triangles - MathBitsNotebook (Geo) (44)

13. Congruent segments have = measure.

14. MN + M1N = MM1

14. Segment Addition Postulate (or whole quantity)

15. MN + MN = DE

15. Substitution

16. 2MN = DE

16. Addition (or combine like terms)

17. MN = ½DE

17. Division (or Multiplication) of Equalities


MidSegments in Triangles - MathBitsNotebook (Geo) (45)

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MidSegments in Triangles - MathBitsNotebook (Geo) (2024)

FAQs

MidSegments in Triangles - MathBitsNotebook (Geo)? ›

"Mid-Segment Theorem": The mid-segment of a triangle, which joins the midpoints of two sides of a triangle, is parallel to the third side of the triangle and half the length of that third side of the triangle.

What are midsegments in triangles? ›

The midsegment of a triangle is a line segment connecting the midpoints of two sides of the triangle. There are three midsegments in each triangle. In a triangle ABC, there is a midsegment connecting AB to BC, a midsegment connecting AB to AC, and a midsegment connecting BC to AC.

What are the centers of a triangle in Mathbitsnotebook? ›

So, how many "centers" does a triangle possess? The Greeks knew of three such "centers", the centroid, the orthocenter, and the circumcenter, referred to as the classical triangle centers.

What is the formula for the midline theorem of a triangle? ›

If a line segment adjoins the mid-point of any two sides of a triangle, then the line segment is said to be parallel to the remaining third side and its measure will be half of the third side. DE = (1/2 * BC).

What is the length of the midsegment? ›

A segment which connects the midpoints of two sides of a triangle are called midsegments of a triangle. The midsegment theorem states that the midsegment of two sides of a triangle is parallel to the third side and the length of the midsegment is half the length of the third side.

What is the midsegment theorem in Mathbits? ›

"Mid-Segment Theorem": The mid-segment of a triangle, which joins the midpoints of two sides of a triangle, is parallel to the third side of the triangle and half the length of that third side of the triangle.

What is the formula for midsegment? ›

The Midsegment Theorem states that the midsegment connecting the midpoints of two sides of a triangle is parallel to the third side of the triangle, and the length of this midsegment is half the length of the third side. So, if D F ¯ is a midsegment of △ A B C , then D F = 1 2 A C = A E = E C and D F ¯ ‖ A C ¯ .

What is the 45 *- 45 *- 90 * triangle theorem? ›

The 45-45-90 triangle rule states that the three sides of the triangle are in the ratio 1:1:\(\sqrt{2}\). So, if the measure of the two congruent sides of such a triangle is x each, then the three sides will be x, x and \(\sqrt{2}x\). This rule can be proved by applying the Pythagorean theorem.

Does a midsegment cut a triangle in half? ›

Midsegments divide the sides of a triangle exactly in half

In this lesson we'll define the midsegment of a triangle and use a midsegment to solve for missing lengths. Hi! I'm krista.

Is the third side of the triangle the length of the midsegment? ›

Triangle Midsegment Theorem

The midsegment theorem states that a line segment connecting the midpoints of any two sides of a triangle is parallel to the third side of a triangle and is half of it.

Are all midsegments congruent? ›

No. Because only 2 midsegments parallel to the congruent legs can be congruent.

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