Cos 7pi 12 Exact Value

Finding the Exact Value of cos(7π/12): A Comprehensive Guide

Determining the exact value of trigonometric functions for angles that aren't standard (like 30°, 45°, 60°) often requires using trigonometric identities and a bit of clever manipulation. Finding the exact value of cos(7π/12) is a great example of this process. This article will guide you through the steps, explaining the underlying principles and providing a deeper understanding of trigonometric identities. We'll break down the problem, explore different approaches, and even delve into the rationale behind each step, ensuring you fully grasp the concept.

Understanding the Problem: cos(7π/12)

Our goal is to find the exact value of cos(7π/12). This angle, 7π/12 radians, is equivalent to 105°. It's not a standard angle found on the unit circle, meaning we can't directly look up its cosine value. Instead, we need to express 7π/12 as a sum or difference of angles whose cosine values are known.

Method 1: Using the Sum-to-Product Formula

One effective approach involves using the sum formula for cosine:

cos(A + B) = cos(A)cos(B) - sin(A)sin(B)

We can express 7π/12 as the sum of two angles whose cosine and sine values are readily available:

7π/12 = π/3 + π/4

Now, let's substitute these values into the sum formula:

cos(7π/12) = cos(π/3 + π/4) = cos(π/3)cos(π/4) - sin(π/3)sin(π/4)

We know the following standard values:

• cos(π/3) = 1/2
• cos(π/4) = √2/2
• sin(π/3) = √3/2
• sin(π/4) = √2/2

Substituting these values into our equation:

cos(7π/12) = (1/2)(√2/2) - (√3/2)(√2/2) = (√2 - √6) / 4

Therefore, the exact value of cos(7π/12) is (√2 - √6) / 4.

Method 2: Using the Half-Angle Formula

Another route to the solution involves utilizing the half-angle formula for cosine:

cos(θ/2) = ±√[(1 + cosθ)/2]

We can express 7π/12 as half of a known angle:

7π/12 = (7π/6) / 2

The sign (±) depends on the quadrant in which 7π/12 lies. Since 7π/12 is in the second quadrant (between π/2 and π), cosine is negative in that quadrant. Therefore, we will use the negative sign.

Now, we need the value of cos(7π/6), which is -√3/2. Substituting into the half-angle formula:

cos(7π/12) = -√[(1 + cos(7π/6))/2] = -√[(1 - √3/2)/2] = -√[(2 - √3)/4] = -√(2 - √3) / 2

This expression doesn't immediately look like our previous result, but we can simplify it further by rationalizing the denominator and manipulating the expression:

Let's square both sides to get rid of the square root:

[cos(7π/12)]² = (2 - √3) / 4

This still needs further simplification. Let's consider the result we obtained in Method 1: [(√2 - √6) / 4]².

[(√2 - √6) / 4]² = (2 - 2√12 + 6) / 16 = (8 - 4√3) / 16 = (2 - √3) / 4

This confirms that both methods lead to the same result, although the form might appear different initially. The algebraic manipulation to show the equivalence fully would involve quite a bit of intricate simplification and isn't very illustrative for a foundational explanation. The key is understanding the different routes to achieve the same solution.

Method 3: Using the Difference Formula

We can also express 7π/12 as a difference of angles:

7π/12 = π/3 - π/4

Using the cosine difference formula:

cos(A - B) = cos(A)cos(B) + sin(A)sin(B)

cos(7π/12) = cos(π/3 - π/4) = cos(π/3)cos(π/4) + sin(π/3)sin(π/4) = (1/2)(√2/2) + (√3/2)(√2/2) = (√2 + √6) / 4

Notice this result differs from our earlier answer. This is because the subtraction formula leads to a different result. The initial expression of 7π/12 as a sum or difference impacts the final answer. There is an error in calculation here. The correct method is to write 7π/12 as π/3 + π/4, as in Method 1, to arrive at the correct answer. This highlights the importance of selecting appropriate angle combinations.

Deep Dive: Why These Methods Work

The success of these methods hinges on the fundamental trigonometric identities. These identities are derived from the unit circle definition of sine and cosine, and they provide powerful tools for manipulating and simplifying trigonometric expressions. The sum-to-product and half-angle formulas are particularly useful when dealing with angles that aren't standard. Understanding these identities provides a much broader understanding of trigonometry and its applications.

Frequently Asked Questions (FAQ)

• Q: Why is it important to find the exact value? A: While calculators can give approximate decimal values, exact values are crucial in theoretical mathematics, advanced calculations, and situations demanding precision.

• Q: Can I use a calculator to check my answer? A: Yes, you can use a calculator to approximate the value of cos(7π/12). The result should be approximately -0.2588, which aligns closely with our exact value of (√2 - √6)/4. However, remember the calculator provides an approximation, while we have determined the exact value.

• Q: Are there other methods to solve this problem? A: While these methods are the most straightforward, other approaches might involve more complex identities or multiple steps. The key is to find a combination of angles that simplify the calculation.

• Q: What if the angle was different? A: The same principles apply. The key is to express the angle as a sum, difference, or multiple of angles with known trigonometric values. The choice of method may change depending on the specific angle.

Conclusion

Finding the exact value of cos(7π/12) requires a strategic approach utilizing trigonometric identities. We've explored three common methods – the sum formula, the half-angle formula, and the difference formula – highlighting the importance of choosing the right formula and understanding the underlying principles. Mastering these techniques empowers you to solve similar problems and deepens your understanding of trigonometry. Remember, the beauty of mathematics lies in the various pathways leading to the same solution, showcasing the elegance and interconnectedness of mathematical concepts. By understanding these methods, you've not only found the exact value but also developed a more comprehensive grasp of trigonometric identities and problem-solving strategies. Remember to always double-check your work and consider the quadrant of the angle when using half-angle or other formulas to ensure the correct sign.