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MATHEMATICS FORM FOUR TOPIC 5: TRIGONOMETRY

) then the trigonometrical ratios are the same as the trigonometrical ratio of 180

^{0}-θ

If θis a reflex angle (180^{0}< θ<270 style="box-sizing: border-box;" sup="">0

) then the trigonometrical ratios are the same as that of θ- 180

^{0}

If θis a reflex angle (270^{0}< θ< 360^{0}), then the trigonometrical ratios are the same as that of 360^{0 }-θ

We have seen that trigonometrical ratios are positive or negative depending on the size of the angle and the quadrant in which it is found.

The result can be summarized by using the following diagram.

Trigonometric Ratios to Solve Problems in Daily Life

Apply trigonometric ratios to solve problems in daily life

Example 1

Write the signs of the following ratios

- Sin 170
^{0}
- Cos 240
^{0}
- Tan 310
^{0}
- sin 30
^{0}

Since 170^{0} is in the second quadrant, then Sin 170^{0 }= Sin (180^{0}-170^{0}) = Sin 10^{0}

∴Sin 170^{0} = Sin 10^{0}

b) Cos 240^{0} = -Cos (240^{0}-180^{0)}= -Cos 60^{0}

Therefore Cos 240^{0}= -Cos 60^{0}

c) Tan 310^{0 }= -Tan (360^{0}-310^{0}) = - Tan 50^{0}

Therefore Tan 310^{0}= -Tan 50^{0}

d) Sin 300^{0}= -sin (360^{0}-300^{0}) = -sin 60^{0}

Therefore sin 300^{0}= - Sin 60^{0}

**Relationship between Trigonometrical ratios**

The above relationship shows that the Sine of angle is equal to the cosine of its complement.

Also from the triangle ABC above

b^{2} = a^{2}+c^{2} (Pythagoras theorem)

Example 2

Given thatA is an acute angle and Cos A= 0.8, find

Example 3

If A and B are complementary angles,

If A and B are complementary angle

Then Sin A = Cos B and Sin B = Cos A

Example 4

Given that θand βare acute angles such that θ+ β= 90^{0} and Sinθ= 0.6, find tanβ

Sines and Cosines of Angles 0 Such That -720°≤ᶿ≥ 720°

Find sines and cosines of angles 0 such that -720°≤ᶿ≥ 720°

**Positive and Negative angles**

An angle can be either positive or negative.

**Positive angle**: is an angle measures in anticlockwise direction from the positive X- axis

**Negative angle**: is an angle measured in clockwise direction from the positive X-axis

**Facts:**

- From the above figure if is a positive angle then the corresponding negative angle to is (- 360
^{0}) or (+ - 360^{0})
- .If is a negative angle, its corresponding positive angle is (360+)

Example 5

Find thecorresponding negative angle to the angle θif ;

Example 6

What is the positive angle corresponding to - 46°?

The angles included in this group are 0^{0}, 30^{0}, 45^{0}, 60^{0}, 90^{0}, 180^{0}, 270^{0}, and 360^{0}

Because the angle 0^{0}, 90^{0}, 180^{0}, 270^{0}, and 360^{0}, lie on the axes then theirtrigonometrical ratios are summarized in the following table.

The ∆ ABC is an equilateral triangle of side 2 units

For the angle 45^{0} consider the following triangle

The following table summarizes the Cosine, Sine, and tangent of the angle 30^{0 }, 45^{0} and 60^{0}

*NB: *The following figure is helpful to remember the trigonometrical ratios of special angles from 0°to 90°

If we need the sines of the above given angles for examples, we only need to take the square root of the number below the given angle and then the result is divided by 2.

Example 7

Find the sine,cosine and tangents of each of the following angles

Example 8

Find the value of θif Cos θ= -½ and θ≤ θ≤ 360°

Since Cos θis – (ve), then θlies in either the second or third quadrants,

Now - Cos (180 –θ= - Cos (θ+180^{0}) = -½= -Cos60^{0}

So θ= 180^{0}-60^{0} = 120^{0} or θ= 180^{0} + 60^{0} = 240^{0}

The Graphs of Sine and Cosine

Draw the graphs of sine and cosine

Consider the following table of value for y=sinθ where θranges from - 360°to 360°

For cosine consider the following table of values

From the graphs for the two functions a reader can notice that sinθand cosθboth lie in the interval -1 and 1 inclusively, that is -1≤sinθ1 and -1≤cosθ≤1 for all values of θ.

The graph of y= tanθis left for the reader as an exercise

*NB: -∞**≤ tanθ**≤∞**the symbol ∞**means infinite*

Also you can observe that both Sinθnd cosθrepeat themselves at the interval of

360°, which means sinθ= sin(θ+360) = sin(θ+2x360^{0}) etc

and Cosθ=(Cosθ+360^{0})= Cos(θ+2x360^{0})

Each of these functions is called a period function with a period 360^{0}

1. Usingtrigonometrical graphs in the interval -360^{0}≤θ≤360^{0}

Example 10

Use the graph of sinθto find the value ofθif

4Sinθ= -1.8 and -360^{0} ≤θ≤360^{0}

So θ= -153^{0}, -27^{0}, 207^{0}, 333^{0}

The graphs of sine and cosine functions

Interpret the graphs of sine and cosine functions

Example 11

Use thetrigonometrical function graphs for sine and cosine to find the value of

- Sin (-40
^{0})
- Cos (-40
^{0})

*Solution*

- Sin (-40
^{0})= - 0.64
- Cos (-40
^{0})= 0.76

The Sine and Cosine Rules

Derive the sine and cosine rules

Consider the triangle ABC drawn on a coordinate plane

From the figure above the coordinates of A, B and C are (0, 0), (c, 0) and(bCosθ, bSinθ) respectively.

Now by using the distance formula

Consider the triangle ABC below

Note that this rule can be started as “In any triangle the side are proportional to the Sines of the opposite angles”

The Sine and Cosine Rules in Solving Problems on Triangles

Apply the sine and cosine rules in solving problems on triangles

Example 12

Find the unknown side and angle in a triangle ABC given that

Find the unknown sides and angle in a triangle ABC in which a= 22.2cmB= 86°and A= 26°

Example 13

Find unknown sides and angles in triangle ABC

Where a=3cm, c= 4cm and B= 30°

Example 14

Find the unknown angles in the following triangle

Exercise 3

1. Given thata=11cm, b=14cm and c=21cm, Find the Largest angle of ΔABC

2. If ABCD is a parallelogram whose sides are 12cm and 16cm what is the length of the diagonal AC if angle B=119°?

3. A and B are two ports on a straight Coast line such that B is 53km east of A. A ship starting from A sails 40km to a point C in a direction E65°N. Find:

- The distance a of the ship from B
- The distance of the ship from the coast line.

4. Find the unknown angles and sides in the following triangle.

5. A rhombus has sides of length 16cm and one of its diagonals is 19cm long. Find the angles of the rhombus.

The Compound of Angle Formulae or Sine, Cosine and Tangent in Solving Trigonometric Problems

Apply the compound of angle formulae or sine, cosine and tangent in solving trigonometric problems

The aim is to express Sin (α±β) and Cos (α±β) in terms of Sinα, Sinβ, Cosαand Cosβ

Consider the following diagram:

For Cos(α±β) Consider the following unit circle with points P and Q on it such that OP,makes angleα with positive x-axis and OQ makes angle βwith positive x-axes.

From the figure above the distance d is given by

Example 15

1. Without using tables find the value of each of the following:

Exercise 4

1. Withoutusing tables, find:

2. Find Sin 225° from (180°+45°)

3. Verify that

- Sin 90° = 1 by using the fact that 90°=45°+45°
- Cos90°=0 by using the fact that 90°=30°+60°

4. Express each of the following in terms of sine, cosine and tangent of acute angles.

5. By using the formula for Sin (A-B), show that Sin (90°-C)=Cos C

...

Thanks for reading **MATHEMATICS FORM FOUR TOPIC 5: TRIGONOMETRY**

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