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Congruence & Similarity | Mathematics for GCSE/IGCSE - Year 11 PDF Download

Congruence

What does congruent mean?

  • In maths, two shapes are congruent if they are identical in shape and size. One may be a reflection, rotation, or translation of the other.
  • One shape being an enlargement of the other means they are not identical in size, hence not congruent.

How do we prove that two shapes are congruent?

  • To prove that two shapes are congruent, you must demonstrate that they have the same shape and size.
  • It is not necessary to show that they are oriented in the same direction.
  • When a shape is reflected, rotated, or translated, its image remains congruent to the original shape.

Similarity

What are similar shapes?

  • Two shapes are similar if they have the same shape and their corresponding sides are proportional.
    • One shape is a scaled version of the other.
  • If two triangles of different sizes have identical angles, they are similar.
    • Other shapes can have the same angles but may not be similar.
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How do we prove that two shapes are similar?

  • To demonstrate that two non-triangular shapes are similar, you need to show that their corresponding sides are proportional.
    • Find the scale factor by dividing the length of one side by the length of the corresponding side on the other shape.
  • If the scale factor is consistent for all corresponding sides, the shapes are similar.
  • If one shape can be proven to be an enlargement of the other, then the two shapes are similar.
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How do we prove that two triangles are similar?

  • To prove that two triangles are similar, you only need to show that their angles are the same.
  • This can be achieved by using angle properties, such as identifying isosceles triangles, vertically opposite angles, and angles on parallel lines.
  • The triangles may appear different and may be oriented differently, so focus on identifying the angles.
    • It can be helpful to sketch both triangles next to each other and facing the same direction.
  • If asked to prove that two triangles are similar, state that corresponding angles in similar triangles are equal and provide a reason for each corresponding equal angle.
    • Often, the triangles may be positioned opposite each other in an hourglass shape, so look for the vertically opposite, equal angles.

Question for Congruence & Similarity
Try yourself:
Which statement accurately describes congruent shapes?
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The document Congruence & Similarity | Mathematics for GCSE/IGCSE - Year 11 is a part of the Year 11 Course Mathematics for GCSE/IGCSE.
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FAQs on Congruence & Similarity - Mathematics for GCSE/IGCSE - Year 11

1. What is the difference between congruence and similarity?
2. How do you prove two shapes are congruent?
Ans. Two shapes are proven to be congruent if all corresponding angles are equal and all corresponding sides are equal in length.
3. Can two similar shapes be congruent?
Ans. Yes, two similar shapes can be congruent if they are both the same size and have the same shape.
4. What are some real-life examples of congruence and similarity?
Ans. Real-life examples of congruence include identical twins, where the two individuals have the same genetic makeup. Similarity can be seen in buildings that have the same architectural design but are different in size.
5. How are congruence and similarity used in geometry?
Ans. Congruence and similarity are important concepts in geometry as they help in determining the relationships between shapes and figures, allowing for accurate measurements and calculations.
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