Triangles are polygons with three sides.
Types of Triangles
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An acute − angled triangle contains no angles greater than 90°. e.g. |
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A right − angled triangle contains an angle of 90°. e.g. |
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| An obtuse − angled triangle contains an angle between 90°and 180°. |  |
A , 
- A scalene triangle contains three sides unequal in length. All angle sizes are different.
- An isosceles triangle is one in which two of the sides and their opposite angles are equal.
- An equilateral triangle is one in which all tree of the sides are equal and each angle is 60°.
- Congruent triangles have the same size and shape.
- Similar triangles have the same shape.
Symmetry of Triangles
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An isosceles triangle has two equal sides and two equal angles .An isosceles triangle has one axis of symmetry. |
e.g. sides: a = b angles: |
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An equilateral triangle is one in which all three sides are of equal length and each interior angle is 60°. An equilateral triangle has three axes of symmetry. |
e.g. sides:
angles: |
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Properties of Triangles
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The interior or inside angles of a triangle always add up to 180°. They are supplementaryangles. |
a + b + c = 180° |
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| An exterior angle of a triangle is equal to the sum of the two opposite interior angles. | w = x + y |  |
| The exterior angles of a triangle always add up to 360°. | m + n + p = 360° |
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- The smallest angle of a triangle is opposite the smallest side.
- The largest angle of a triangle is opposite the largest side.
