Name sides and angles in 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

#### 10D.T2.01

Determine, through investigation , the relationship between the ratio of two sides in a right triangle and the ratio of the two corresponding sides in a similar right triangle, and define the sine, cosine, and tangent ratios