top of page

Secondary 3 Math: Top 3 Common Mistakes & Questions in Congruence and Similarity

Updated: Jun 25

Congruence and Similarity

Congruence and similarity are important aspects of geometry, and their underlying principles should be grasped by students to discern whether shapes exhibit congruence or similarity. These concepts are typically applied in the context of triangles in the E-Math syllabus.

 This article delves into the meanings of congruence and similarity, outlines the rules for determining these properties, and highlights the most common mistakes and questions students encounter in the exams.

What Do Congruence and Similarity Mean?

Congruent figures are those that share the same shape and size, whereas similar figures share the same shape but not necessarily the same size. To ascertain if two triangles exhibit similarity and congruence, several tests can be applied. Understanding these tests is crucial for distinguishing between congruent and similar shapes.

What Do Congruence and Similarity Mean

Congruence Rules/Tests

To determine if two triangles are congruent, the following tests can be applied:

Congruence Rules/Tests

Top 3 Common Mistakes in Congruence and Similarity

Understanding the rules is just fundamental. Applying them correctly is where many students falter. Here are the top three mistakes students make in congruence and similarity rules.

Top 3 Common Mistakes in Congruence and Similarity

1. Incorrect Application of Congruence Rules

One of the most frequent errors students make is the misapplication of congruence rules, especially SAS (Side-Angle-Side) and ASA (Angle-Side-Angle). This mistake often stems from a misunderstanding of the specific conditions under which these rules are valid or mixing up the rules. 

For the SAS rule, it is crucial to ensure that the two sides and the included angle of one triangle exactly correspond to the two sides and the included angle of another triangle. Similarly, for the ASA rule, the two angles and the included side of one triangle must precisely match the corresponding two angles and the included side of another triangle. Misinterpretation occurs when students incorrectly identify the included angle or the sides that should directly encompass the angle for the SAS rule, or when they fail to verify that the side being compared is indeed the one flanked by the two relevant angles for the ASA rule.

Incorrect Application of Congruence Rules

2. Overlooking Transformational Properties

Overlooking transformational properties can result in incorrect conclusions about the relationships between figures. Transformations such as translations (sliding), reflections (flipping), rotations (turning), and dilations (resizing) are ways figures can be manipulated while preserving their congruence or establishing their similarity. Students who ignore these transformational aspects often struggle to accurately identify congruent and similar figures.

For instance, a student might fail to recognise that a figure, though rotated, remains congruent to its original shape. An example of this is two rotated triangles, which, despite their new orientation, are still congruent.

Neglecting Diagram Analysis

3. Neglecting Diagram Analysis

Rushing through problems without thoroughly examining and annotating diagrams is a common pitfall. Essential details crucial for establishing similarity between triangles, such as congruent angles or proportional side lengths, can be easily missed. Students often overlook marked angles and sides indicating congruence. Systematically identifying and marking corresponding elements on diagrams is vital for accurate analysis.

Common Question Types in Congruence and Similarity

Common Question Types in Congruence and Similarity

To reinforce the understanding of congruence and similarity, let's explore common question types students encounter and the proper approach to solving them.

1. Proving Congruence

Proving Congruence

Example: A rhombus, ABCD, is illustrated in the diagram above. Explain clearly why ADE and CDE are congruent.

In this question, students must be able to discern that all sides of a rhombus are equal. Therefore, AD=CD. ADE and CDE also share a common side, DE. Lastly, one is to recognise that ADE=CDE because line DB bisects ADC. Thus, by SAS congruency, ∆ADE≡∆CDE.

2. Proving Similarity

Proving Similarity

Example: In the triangle ABC, AD=7 cm, DB=2 cm, AE=6 cm. Also, ∠ABC=∠AED.

a. Show that ΔAED and ΔABC are similar.

b. Find the value of EC.

For part a, AED=ABC (given), and EAD=BAC (common angles). Considering that these two angles are equal between both triangles, it follows that the remaining angle must also be the same. Hence, based on the Angle-Angle (AA) similarity criterion, triangles ∆AED and ∆ABC exhibit similarity.

For part b, since we have proven that the 2 triangles are similar,

2 triangles are similar
Mastering congruence and similarity

Mastering congruence and similarity involves more than memorising definitions and theorems; it requires careful application and attention to the details that distinguish these concepts. By understanding and correctly applying the congruence and similarity rules, students can avoid common mistakes and tackle various types of questions with confidence. Practice with different question types and meticulous diagram analysis will enhance familiarity and proficiency in using knowledge of congruence and similarity in various contexts.

Unlock Your Child’s Potential with AGrader Learning Centre's top-notch E-Math and A-Math tuition programmes today. Our meticulously curated weekly lessons, aligned with the latest MOE syllabus, are designed to stay ahead of the school curriculum. From Sec 1 to O-Level Math, our dedicated teachers are here to ensure your child's success.

Unlock Your Child’s Potential with AGrader Learning Centre's

Additionally, we offer the exclusive EverLoop Improvement System for unlimited after-class revision sessions, both in-person and online, to address learning gaps. This comprehensive support system is complimentary for AGrader students.

Join Our AGrader Community Today! (Free for all AGrader & Non-AGrader students)

Join our Telegram and WhatsApp channels to gain access to more study tips, free downloads, and much more for all subjects. 

- WhatsApp channel: 

- Telegram channel: 



bottom of page