NZ Level 6 (NZC) Level 1 (NCEA)
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Trigonometric Ratios
Lesson

 

Right-angled triangle

Trigonometric Ratios

A ratio is a statement of a mathematical relationship comparing two quantities, often represented as a fraction. If we consider an angle $\theta$θ in a right-angled triangle, we can construct various ratios to compare the lengths of the sides. Special relationships that exist in right-angled triangles are called trigonometric ratios.

Through investigation we can see that there is a definite relationship between the angles in a right-angled triangle and the ratio of sides. We can use these trigonometric ratios to find unknown angles and sides of a triangle.

There are 3 basic trigonometric ratios that relate sides and angles together.  They have the special names of tangent, sine and cosine.  The names of the relationships date back to 499AD, with ties to Latin and Sanskrit.  Actually the word sine is thought to be derived from a translation gone wrong! Regardless the names for these relationships have stuck.  

We often shorten the names tangent, sine and cosine to tan, sin and cos respectively. 

Trigonometric ratios

       $\sin\theta$sinθ = $\frac{Opposite}{Hypotenuse}$OppositeHypotenuse  = $\frac{b}{c}$bc

       $\cos\theta$cosθ = $\frac{Adjacent}{Hypotenuse}$AdjacentHypotenuse = $\frac{a}{c}$ac

       $\tan\theta$tanθ = $\frac{Opposite}{Adjacent}$OppositeAdjacent = $\frac{b}{a}$ba

Here is a picture of the above relationships, and for some people the mnemonic of SOHCAHTOA at the bottom is helpful to remember the sides that apply to the different ratios of sine, cosine and tangent.

This applet will let you explore different trigonometric ratios, particularly which sides are needed for a particular ratio.  If you would like to see this applet in action, watch this video.  

Another relationship

Let's have a look at just one more special relationship.

If we know the sine and cosine ratios for a particular angle, 

$\sin\theta=\frac{Opposite}{Hypotenuse}$sinθ=OppositeHypotenuse

$\cos\theta=\frac{Adjacent}{Hypotenuse}$cosθ=AdjacentHypotenuse

Then we can construct a new relationship for $\text{sine }\div\text{cosine }$sine ÷​cosine

$\frac{\sin\theta}{\cos\theta}$sinθcosθ $=$= $\frac{\left(\frac{Opposite}{Hypotenuse}\right)}{\left(\frac{Adjacent}{Hypotenuse}\right)}$(OppositeHypotenuse)(AdjacentHypotenuse)
  $=$= $\frac{Opposite}{Hypotenuse}\times\frac{Hypotenuse}{Adjacent}$OppositeHypotenuse×HypotenuseAdjacent
  $=$= $\frac{Opposite}{Adjacent}$OppositeAdjacent
  $=$= $\tan\theta$tanθ

 

Algebraically we have just shown that $\tan\theta=\frac{\sin\theta}{\cos\theta}$tanθ=sinθcosθ

That is, the tangent ratio of an angle is the same as dividing its sine ratio by its cosine ratio.

Here are some worked examples.

Question 1

Write down the ratio represented by $\cos\theta$cosθ.

Question 2

Find the value of $\tan\theta$tanθ within $\triangle ABC$ABC.

 

Question 3

Consider the triangle in the figure. If $\sin\theta=\frac{4}{5}$sinθ=45:

  1. Which angle is represented by $\theta$θ?

    $\angle BAC$BAC

    A

    $\angle BCA$BCA

    B

    $\angle ABC$ABC

    C

    $\angle BAC$BAC

    A

    $\angle BCA$BCA

    B

    $\angle ABC$ABC

    C
  2. Find the value of $\cos\theta$cosθ.

  3. Find the value of $\tan\theta$tanθ.

 

 

 

 

 

 

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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