8.3 - Trigonometric Ratios of Some Specific Angles
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Understanding Trigonometric Ratios of 45°
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Today we will explore the trigonometric ratios for 45°. Can anyone tell me about the properties of a triangle when one of the angles is 45°?
In a right triangle, if one angle is 45°, the other angle must also be 45°.
Exactly! This creates an isosceles right triangle where the two sides are equal. Now, if we denote those sides as 'a', how would we calculate the hypotenuse?
We use the Pythagorean theorem: AC² = AB² + BC², which means AC = a√2.
Correct! Now using this, can anyone derive sin 45°, cos 45°, and tan 45°?
Yes! Sin 45° = a / (a√2) = 1/√2, which is √2/2. Cos 45° will be the same, and tan 45° = 1.
Perfect! Remember the acronym 'SCC' for 'Sine, Cosine, Complementary' to help you recall that sin and cos are equal at 45°.
Let's summarize: For 45°, we have sin 45° = √2/2, cos 45° = √2/2, and tan 45° = 1.
Exploring 30° and 60° Trigonometric Ratios
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Now, let’s discuss 30° and 60°. Who can tell me the relationship between these angles?
They are complementary angles, meaning they add up to 90°.
Exactly! We can derive their ratios using an equilateral triangle. If each side is 2a, what can we conclude?
We draw a height from one vertex to the opposite side, creating two 30-60-90 triangles.
Well done! In this scenario, how do we calculate sin 30°?
Sin 30° = opposite / hypotenuse, which is a / 2a = 1/2.
And for sin 60°?
Sin 60° = √3/2 because the opposite side is a√3 and the hypotenuse is 2a.
Great! The key angles to remember: sin 30° = 1/2, sin 60° = √3/2. Now let’s summarize.
Understanding Trigonometric Values for 0° and 90°
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Let’s finalize by discussing sin and cos for 0° and 90°. What can we observe as we approach these angles?
As the angle approaches 0°, the opposite side length approaches 0, which makes sin 0° = 0.
Exactly! And what about cos 0°?
Cos 0° = 1 since the hypotenuse remains unchanged.
Right! Now, as we move towards 90°, what happens to sin and cos?
Sin 90° becomes 1, and cos 90° becomes 0 as the triangle squashes.
Well summarized! For 0°: sin 0° = 0, cos 0° = 1; and for 90°: sin 90° = 1, cos 90° = 0. Let’s recap that!
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
Students will learn to calculate trigonometric ratios for specific angles like 0°, 30°, 45°, 60°, and 90°. This section derives these values through geometric constructions and the properties of right triangles, emphasizing the significance in trigonometry.
Detailed
Detailed Summary
In this section, we delve into the trigonometric ratios of key angles of 0°, 30°, 45°, 60°, and 90° through the application of right triangle properties and geometric constructions. We begin with calculating ratios for 45°, highlighting that in a right triangle where both angles are 45°, the legs are equal in length. By utilizing the Pythagorean theorem, we formulate the ratios, demonstrating that:
- sin 45° = cos 45° = √2/2
- tan 45° = 1
Next, we explore the ratios for angles 30° and 60° using an equilateral triangle. By drawing a perpendicular from one vertex, we create two right triangles, showing that:
- sin 30° = 1/2 and sin 60° = √3/2
- cos 30° = √3/2 and cos 60° = 1/2
- tan 30° = 1/√3 and tan 60° = √3
Finally, we address the limits of trigonometric functions as angles approach 0° and 90°, defining values for:
- sin 0° = 0, cos 0° = 1
- sin 90° = 1, cos 90° = 0
The section concludes by summarizing the values of all calculated trigonometric ratios in a convenient table for reference.
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Trigonometric Ratios of 45°
Chapter 1 of 3
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Chapter Content
In Δ ABC, right-angled at B, if one angle is 45°, then the other angle is also 45°, i.e., ∠ A = ∠ C = 45° (see Fig 8.14).
So, BC = AB (Why?)
Now, Suppose BC = AB = a.
Then by Pythagoras Theorem, AC² = AB² + BC² = a² + a² = 2a², and, therefore, AC = a√2.
Using the definitions of the trigonometric ratios, we have:
- sin 45° = BC/AC = a / (a√2) = 1/√2
- cos 45° = AB/AC = a / (a√2) = 1/√2
- tan 45° = BC/AB = a/a = 1
- cosec 45° = 1/sin 45° = √2,
- sec 45° = 1/cos 45° = √2,
- cot 45° = 1/tan 45° = 1.
Detailed Explanation
In a right triangle where one angle is 45°, both other angles must also be 45° making this a special isosceles triangle. The lengths of the two legs (BC and AB) are equal. By using the Pythagorean theorem, we find the length of the hypotenuse (AC) as 'a√2', given that both legs are of equal length 'a'. The trigonometric ratios can then be calculated, revealing that sine and cosine of 45° are both equal to '1/√2', which simplifies to approximately 0.707.
Examples & Analogies
Imagine you're climbing a staircase where each step makes a 45° angle with the ground. Since every time you go up one step (the height), you also move out one step (the horizontal distance), both distances are equal. If you used a ruler to measure these steps, both would be the same, reinforcing that the sine and cosine of those angles are equal.
Trigonometric Ratios of 30° and 60°
Chapter 2 of 3
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Chapter Content
Let us now calculate the trigonometric ratios of 30° and 60°. Consider an equilateral triangle ABC. Since each angle in an equilateral triangle is 60°, therefore, ∠ A = ∠ B = ∠ C = 60°.
Draw the perpendicular AD from A to the side BC (see Fig. 8.15).
Now, Δ ABD ≅ Δ ACD (Why?) Thus, BD = DC and ∠ BAD = ∠ CAD (CPCT).
Now observe that:
Δ ABD is a right triangle, right-angled at D with ∠ BAD = 30° and ∠ ABD = 60°.
As you know, for finding the trigonometric ratios, we need to know the lengths of the sides of the triangle. So, let us suppose that AB = 2a.
Then, BD = BC= a/2 and AD² = AB² – BD² = (2a)² – (a)² = 3a², Therefore, AD = a√3. Now, we have:
- sin 30° = BD/AB = a / (2a) = 1/2,
- cos 30° = AD/AB = (a√3)/(2a) = √3/2,
- tan 30° = BD/AD = a/(a√3) = 1/√3.
Detailed Explanation
In the triangle, we derive the lengths needed to calculate the trigonometric ratios for 30° and 60°. Since we form a right triangle from half of the equilateral triangle, we can use properties of the angles and the lengths of the sides to derive sine and cosine values specifically tied to these angles. The side relationships reveal that 30° has a sine of 1/2 and a cosine of √3/2, while the tangent is 1/√3, making them interconnected using the sides drawn from a common height.
Examples & Analogies
Think of a tall tree. If you measure the height of the tree and find a point where the angle from your eyes to the top of the tree is 30°, you will be standing at half the tree's height (where the sine values apply). A triangle made from you to the top of the tree forms similar ratios that reflect these trigonometric values.
Trigonometric Ratios of 0° and 90°
Chapter 3 of 3
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Chapter Content
Let us see what happens to the trigonometric ratios of angle A, if it is made smaller and smaller in the right triangle ABC (see Fig. 8.16), till it becomes zero. As ∠ A gets smaller and smaller, the length of the side BC decreases. The point C gets closer to point B, and finally when ∠ A becomes very close to 0°, AC becomes almost the same as AB (see Fig. 8.17).
When ∠ A is very close to 0°, BC gets very close to 0 and so the value of sin A = BC/AC is very close to 0. Also, when ∠ A is very close to 0°, AC is nearly the same as AB and so the value of cos A = AC/AB is very close to 1. This helps us to see how we can define the values of sin A and cos A when A = 0°.
We define: sin 0° = 0 and cos 0° = 1. Using these, we have :
- tan 0° = sin 0° / cos 0° = 0,
- cot 0° is not defined,
- sec 0° = 1/cos 0° = 1,
- cosec 0° is not defined.
Detailed Explanation
As angle A approaches 0°, we intuitively understand that the height from the base converges to a line, leading to near-zero sine while the cosine ratio remains close to 1. These limits help us differenciate between values of these trigonometric functions at boundaries. At 0°, we can establish known values: sin 0°=0 denotes the absence of height, while cos addresses the full horizontal stretch.
Examples & Analogies
Picture diving from a diving board. As you lean forward to dive, you angle towards the water (θ approach 0°). Ultimately, when you dive straight, you're close to the horizontal which represents cos (like being flat) and your height drops to zero, portrayed as sin (falling into the water).
Key Concepts
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Trigonometric Ratios: Defined for angles using sides of right triangles.
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Sine of an Angle: Ratio of opposite side to hypotenuse.
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Cosine of an Angle: Ratio of adjacent side to hypotenuse.
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Values for Special Angles: Specific sine, cosine, and tangent values for 0°, 30°, 45°, 60°, and 90°.
Examples & Applications
Finding sin 30°: By constructing a right triangle, sin 30° = 1/2.
Calculating tan 45°: Using the definition, tan 45° = 1 since both opposing sides are equal.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
At 30° and 60° in trigonometry's dance, sin and cos take their chance.
Stories
Imagine a triangle where an angle shines bright at 45°. The pairs of sides represent equal delight!
Memory Tools
Acronym SOH-CAH-TOA: Sine = Opposite/Hypotenuse, Cosine = Adjacent/Hypotenuse, Tangent = Opposite/Adjacent.
Acronyms
For 45° remember SCC
Sine
Cosine
Correspond equally.
Flash Cards
Glossary
- Trigonometric Ratios
Relational ratios of the sides of a right triangle, defining the sine, cosine, tangent, cosecant, secant, and cotangent functions.
- Sine
Ratio of the length of the opposite side to the hypotenuse in a right triangle.
- Cosine
Ratio of the length of the adjacent side to the hypotenuse in a right triangle.
- Tangent
Ratio of the length of the opposite side to the adjacent side in a right triangle.
- Cosecant
Reciprocal of sine, or 1/sin.
- Secant
Reciprocal of cosine, or 1/cos.
- Cotangent
Reciprocal of tangent, or 1/tan.
Reference links
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