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Triangles are plane shapes which lie in one plane. They have area but not volume. Shapes that have only straight sides are called polygons. Triangles are polygons. The sum of the angles in any triangle is 180º.

 

 

Examples

 

EQUILATERAL		all sides are equal and all angles are equal to 60º
ISOSCELES		*two sides are equal *angles opposite the equal sides are equal
SCALENE		*no sides are equal *all angles are different
ACUTE-ANGLED		all angles are less than 90º(acute)
RIGHT-ANGLED		one of the three angles of the triangle is a right-angle
OBTUSE-ANGLED		*one of the angles of the triangle is greater than 90º(obtuse) *the longest side is opposite the largest angle

 

Have a Go Problem 1 Calculate the size of the unknown angle in this triangle: Problem 2

 

 

Practice Questions Question 1 A right-angled triangle has another angle of 36º. What size is the third angle? Answer 54 degrees Question 2 A particular triangle has one obtuse angle of 107º and an acute angle of 45º. What shape will the third triangle be and how many degrees?   Answer acute, 28 degrees Question 3      An equilateral triangle has one side 45cm. What size will the angle opposite that side be?   Answer 60 degrees Question 4 What shaped triangle is used to design an A-framed house usually?   Answer Isosceles

 

 

Solution 1 back to Have a Go

Solution 2       back to Have a Go

 

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