Mathematics
Grade 8
15 min
Side lengths and angle measures of congruent figures
Side lengths and angle measures of congruent figures
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1
Introduction & Learning Objectives
Learning Objectives
Define congruence in terms of rigid transformations.
Identify corresponding sides and corresponding angles of congruent figures.
Write a correct congruence statement for two congruent polygons.
State that corresponding sides of congruent figures are equal in length.
State that corresponding angles of congruent figures are equal in measure.
Use the properties of congruence to find unknown side lengths and angle measures.
Ever wonder how a factory can produce thousands of identical puzzle pieces that fit together perfectly? 🧩 It's all about making sure the pieces are congruent!
This tutorial explores the fundamental property of congruent figures: if two figures are congruent, their matching parts are identical. You will learn how to use this powerful...
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Key Concepts & Vocabulary
TermDefinitionExample
Congruent FiguresTwo geometric figures are congruent if they have the exact same size and shape. This means one figure can be mapped onto the other through a sequence of rigid transformations.If you can slide, flip, or turn Triangle A so it fits perfectly on top of Triangle B, then Triangle A and Triangle B are congruent.
Rigid Transformation (Isometry)A transformation (like a translation, reflection, or rotation) that preserves the size and shape of a figure. The distances between points and the measures of angles remain the same.Translating a square 5 units to the right does not change its side lengths or its 90-degree angles, so it's a rigid transformation.
Corresponding PartsThe sides and angles in two congruent figures that are in the same relative position...
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Core Formulas
Corresponding Sides of Congruent Figures are Equal
If polygon ABCDE \cong polygon FGHIJ, then the lengths of corresponding sides are equal. For example, AB = FG, BC = GH, and so on.
Use this rule when you know two figures are congruent and you need to find a missing side length. Find the corresponding side in the other figure; their lengths will be equal.
Corresponding Angles of Congruent Figures are Equal
If polygon ABCDE \cong polygon FGHIJ, then the measures of corresponding angles are equal. For example, m\angle A = m\angle F, m\angle B = m\angle G, and so on.
Use this rule when you know two figures are congruent and you need to find a missing angle measure. Find the corresponding angle in the other figure; their measures will be equal.
CPCTC (Conceptual Rule)
Corr...
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Challenging
Given △LMN ≅ △RST. If m∠M = (5x - 10)° and m∠S = (3x + 20)°, what is the measure of ∠S?
A.15°
B.65°
C.85°
D.95°
Challenging
A regular hexagon ABCDEF is divided into six congruent equilateral triangles, with all triangles meeting at the center point O. If the length of side AB is 4 cm, what is the length of the segment AO?
A.2 cm
B.6 cm
C.8 cm
D.4 cm
Challenging
Triangle XYZ has m∠Y = 50°. It is first rotated 180° about the origin, and then translated 3 units to the right to create triangle X''Y''Z''. What is the measure of ∠Y''?
A.40°
B.50°
C.130°
D.It cannot be determined.
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