NZ Level 6 (NZC) Level 1 (NCEA)
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Sides of a Right-Angled Triangle
Lesson

We have already seen which of the sides in a right-angled triangle is the hypotenuse.  

If we have another angle indicated (like $\theta$θ in the diagram below) then we can also label the other two sides with two special names.

Opposite Side - is the name given to the side opposite the angle in question

Adjacent Side - is the name given to the side adjacent (next to) the angle in question.
 

Have a look at these triangles that I have named below.  Note how the sides adjacent, opposite and hypotenuse are also abbreviated to A, O and H. 

 

Let's have a look at these worked examples.

Question 1

Which of the following is the opposite side to angle $\theta$θ?

  1. $AB$AB

    A

    $BC$BC

    B

    $AC$AC

    C

    $AB$AB

    A

    $BC$BC

    B

    $AC$AC

    C

Question 2

Which of the following is the adjacent side to angle $\theta$θ?

  1. $AB$AB

    A

    $BC$BC

    B

    $AC$AC

    C

    $AB$AB

    A

    $BC$BC

    B

    $AC$AC

    C

Question 3

A driver glances up at the top of a building.

  1. True or false: According to the angle A, the height of the building is the opposite side.

    True

    A

    False

    B

    True

    A

    False

    B
  2. True or false: According to the angle A, the distance from the driver to the building would be the opposite side.

    True

    A

    False

    B

    True

    A

    False

    B

 

 

 

Outcomes

GM6-6

Use trigonometric ratios and Pythagoras’ theorem in two and three dimensions

91031

Apply geometric reasoning in solving problems

91032

Apply right-angled triangles in solving measurement problems

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