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Revision Guide
Explore the properties, types, and theorems related to triangles, including congruence and similarity, to solve geometric problems effectively.
Triangles - Quick Look Revision Guide
Your 1-page summary of the most exam-relevant takeaways from Mathematics.
This compact guide covers 20 must-know concepts from Triangles aligned with Class X preparation for Mathematics. Ideal for last-minute revision or daily review.
Key Points
Define similar figures.
Two figures are similar if they have the same shape but not necessarily the same size. All corresponding angles are equal, and sides are proportional.
AAA Similarity Criterion.
If in two triangles, corresponding angles are equal, then they are similar. This is known as the Angle-Angle-Angle (AAA) criterion.
SSS Similarity Criterion.
If in two triangles, sides of one are proportional to the sides of the other, then they are similar. This is the Side-Side-Side (SSS) criterion.
SAS Similarity Criterion.
If one angle of a triangle is equal to one angle of another and the including sides are proportional, the triangles are similar. This is the Side-Angle-Side (SAS) criterion.
Basic Proportionality Theorem (Thales).
A line parallel to one side of a triangle divides the other two sides proportionally. It's fundamental for proving similarity.
Converse of BPT.
If a line divides two sides of a triangle proportionally, it is parallel to the third side. Useful for proving lines parallel.
Pythagoras Theorem.
In a right-angled triangle, the square of the hypotenuse equals the sum of squares of the other two sides. Formula: AB² + BC² = AC².
Area of Similar Triangles.
The ratio of areas of two similar triangles is equal to the square of the ratio of their corresponding sides.
Criteria for Congruency vs Similarity.
Congruent triangles are equal in shape and size; similar triangles are same in shape but size may vary. All congruent triangles are similar, not vice versa.
Altitude and Median in Similar Triangles.
In similar triangles, corresponding altitudes and medians are proportional to the corresponding sides.
Right-angled Triangle Similarity.
If a perpendicular is drawn from the right angle to the hypotenuse, the two triangles formed are similar to each other and to the original triangle.
Use of Similarity in Real Life.
Similarity is used in scaling objects, like maps or models, where dimensions are proportionally reduced or enlarged.
Solving Problems with Similarity.
Identify similar triangles using criteria, then use proportionality of sides to find unknown lengths or heights.
Angle Bisector Theorem.
The angle bisector of a triangle divides the opposite side in the ratio of the adjacent sides.
Mid-point Theorem.
The line segment joining mid-points of two sides of a triangle is parallel to the third side and half its length.
Converse of Mid-point Theorem.
If a line is drawn through the mid-point of one side of a triangle parallel to another, it bisects the third side.
Equilateral Triangles are Similar.
All equilateral triangles are similar as their angles are equal (60° each) and sides are in proportion.
Isosceles Triangle Similarity.
Two isosceles triangles are similar if their vertex angles are equal or base angles are equal.
Perimeter of Similar Triangles.
The ratio of the perimeters of two similar triangles is equal to the ratio of their corresponding sides.
Misconception: All Circles are Congruent.
All circles are similar but not congruent unless their radii are equal. Similarity requires same shape, not size.
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