[Maths Class Notes] on Trigonometry Angles Pdf for Exam

Trigonometry Angle Measures

The length of the sides of triangles affects the angles formed by the triangles too. We  know,

These two principles applied to the triangles gives

• All three angles of an equilateral triangle are equal to 600.

• The two angles opposite to the two equal sides are equal.

• All three angles of the scalene triangles are different.

Some Definitions

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Here a is the altitude, b is the base and ‘h’ is the hypotenuse. We can define some very useful ratios using sides and the angle θ. 

(θ) = [frac{text{opposite}}{text{hypotenuse}} side]

[frac{a}{h} = Cosine(Theta) = frac{text{adjacent}}{text{hypotenuse}} side]

[frac{b}{a} = Tan(Theta) = frac{text{opposite}}{text{adjacent}} side]

We can also define their reciprocals as 

[frac{h}{b} = frac{1}{sin(Theta)} = cosec(Theta) = frac{text{hypotenuse}}{text{opposite}} side]

[frac{h}{a} = frac{1}{cos(Theta)} = sec(Theta) = frac{text{hypotenuse}}{text{adjacent}} side]

[frac{a}{b} = frac{1}{tan(Theta)} = cot(Theta) = frac{text{adjacent}}{text{opposite}} side]

Out of these cosine(θ) ,sine (θ) and tan(θ) are more important trigonometric angles.

Here θ is usually expressed in radians. 

180° = ? radians

Two Important Right Triangles

Suppose we have an equilateral triangle of a side of length 1 unit. Drawing an angle bisector towards the base will equally divide the base into ½ unit lengths. We have now obtained two right angles with hypotenuse equal to 1 unit and the base equal to ½ units. We also know that the angle opposite to hypotenuse is 900 and the angle opposite to altitude is 600. Hence it is clear that the other angle is 300. Using the Pythagoras theorem we obtain the value of altitude as [frac{sqrt{3}}{2}] units.

If we try to define the above ratios for the two angles we obtain

• Cos(60) = cos(?/3) = [frac{sqrt{3}}{2}]

• Sin(60) =  sin(?/3) = [frac{1}{2}]

• Tan(60) = tan(?/3) = [frac{1}{sqrt{3}}]

Similarly, we can write,

• Cos(30) =cos(?/6) = [frac{1}{2}]

• Sin(30) = sin(?/6) = [frac{sqrt{3}}{2}]

• Tan(30) = tan(?/6) = [sqrt{3}]

Let us do the same with an isosceles triangle with two angles equal to 450  two sides equal to 1. We obtain two right triangles with two angles equal to 450. Here we obtain

• Cos(45) =cos(?/4) = [frac{1}{sqrt{2}}]

• Sin(45) = sin(?/4) = [frac{1}{sqrt{2}}]

• Tan(45) = tan(?/4) = 1

These values are really helpful in various calculations. Similarly, there are few trigonometric angles which if known can predict the nature of solutions to various problems.