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4. The Right Triangle and Applications

Many problems involve right triangles. We often need to use the trigonometric ratios to solve such problems.

Example 1 - Finding the Height

Find h for the given triangle.

2.500 km h

Triangle with unknown height h.


`sin 15.70^"o" = h/2.500`

`h = 2.500\ sin 15.70^"o"`

`= 0.6765\ "km"`

Example 2 - Solving Triangles

Solve the triangle ABC, given that `A = 35°` and `c = 15.67`.

Triangle sides a,b,c and angles A,B,C.


To "solve" a triangle means to find the unknown sides and angles.

We have, for the values given:

In this example, we need to find side lengths a and b and angle B. Note C = 90o.

`sin 35^"o" = a/c = a/15.67`

So a = 15.67 sin 35o = 8.99

`cos 35^"o"=b/c=b/15.67`

So b = 15.67 cos 35o = 12.84

Angle B = 90o − 35o = 55o.

So a = 8.99, b = 12.84 and B = 55o. We have found all the unknowns.

Angles of Elevation and Depression

In surveying, the angle of elevation is the angle from the horizontal looking up to some object:

Angle of elevation

The angle of depression is the angle from the horizontal looking down to some object:

Angle of depression

Example 3

The angle of elevation of an aeroplane is `23°`. If the aeroplane's altitude is `2500\ "m"`, how far away is it?

Cessna trigonometry application


Let the distance be x. Then `sin 23^"o"=2500/x`

`x=2500/(sin 23^"o")=6400\ "m"`

Example 4

You can walk across the Sydney Harbour Bridge and take a photo of the Opera House from about the same height as top of the highest sail.

opera house

This photo was taken from a point about `500\ "m"` horizontally from the Opera House and we observe the waterline below the highest sail as having an angle of depression of `8°`. How high above sea level is the highest sail of the Opera House?


opera house solution

This is a simple tan ratio problem.

`tan 8^"o" = h/500`


`h= 500\ tan 8° = 70.27\ "m"`.

So the height of the tallest point is around `70\ "m"`.

[The actual height is `67.4\ "m"`.]


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