Triangles are shown in 341:3. Right-angled triangles (A) have two sides at right angles to
each other, the third side is known as the 'hypotenuse' and the angles so formed by this line are equal in sum to one right angle, i.e. 45° each. Equilateral triangles (B) have three equal sides and therefore three equal angles of 60°. Isosceles triangles (C) have two equal sides and two equal angles, and scalene triangles—acute scalene (D) and obtuse scalene (E)—have three unequal sides and therefore three unequal angles.
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