Cos 2x Sin X 0

Solving cos(2x)sin(x) = 0: A Comprehensive Guide

This article provides a detailed explanation of how to solve the trigonometric equation cos(2x)sin(x) = 0. We'll explore various methods, delve into the underlying mathematical principles, and address frequently asked questions. Understanding this equation is crucial for anyone studying trigonometry, pre-calculus, or calculus, as it involves fundamental trigonometric identities and solving techniques. This comprehensive guide aims to not just provide the solution but also to foster a deeper understanding of the concepts involved.

Introduction: Understanding the Equation

The equation cos(2x)sin(x) = 0 presents a classic example of a trigonometric equation requiring the application of trigonometric identities and the understanding of periodic functions. The equation states that the product of cos(2x) and sin(x) equals zero. This implies that at least one of the factors, cos(2x) or sin(x), must be equal to zero. This seemingly simple equation opens doors to a deeper exploration of trigonometric solutions and their periodic nature. We will use this equation as a stepping stone to understand broader concepts in trigonometry.

Method 1: Utilizing the Zero Product Property

The zero product property states that if the product of two factors is zero, then at least one of the factors must be zero. We can apply this directly to our equation:

cos(2x)sin(x) = 0

This means either cos(2x) = 0 or sin(x) = 0 (or both). We will solve each equation separately.

Solving cos(2x) = 0:

The cosine function equals zero at odd multiples of π/2. Therefore:

2x = (2n + 1)π/2, where n is an integer.

Dividing by 2, we get:

x = (2n + 1)π/4, where n is an integer.

This gives us solutions like π/4, 3π/4, 5π/4, 7π/4, and so on, within one period of the cosine function (0 to 2π).

Solving sin(x) = 0:

The sine function equals zero at integer multiples of π. Therefore:

x = nπ, where n is an integer.

This gives us solutions like 0, π, 2π, -π, -2π, and so on.

Combining the Solutions:

The complete solution set for cos(2x)sin(x) = 0 includes all the solutions we found for both cos(2x) = 0 and sin(x) = 0. Therefore, the general solution is:

x = (2n + 1)π/4 and x = nπ, where n is an integer.

Method 2: Using Trigonometric Identities

We can also approach this problem by employing trigonometric identities to simplify the equation. One useful identity is the double angle formula for cosine:

cos(2x) = 1 - 2sin²(x) = 2cos²(x) - 1

Substituting the first identity into our original equation, we get:

(1 - 2sin²(x))sin(x) = 0

This simplifies to:

sin(x) - 2sin³(x) = 0

Factoring out sin(x), we get:

sin(x)(1 - 2sin²(x)) = 0

This equation is now in a similar form to our first approach. We have two factors: sin(x) and (1 - 2sin²(x)). Setting each factor to zero gives us:

sin(x) = 0 or 1 - 2sin²(x) = 0

The first equation, sin(x) = 0, gives us the same solution as before: x = nπ, where n is an integer.

The second equation, 1 - 2sin²(x) = 0, simplifies to:

sin²(x) = 1/2

Taking the square root of both sides, we get:

sin(x) = ±√(1/2) = ±1/√2 = ±√2/2

This means sin(x) = √2/2 or sin(x) = -√2/2. The solutions for these are:

x = π/4 + 2nπ, x = 3π/4 + 2nπ, x = 5π/4 + 2nπ, x = 7π/4 + 2nπ, where n is an integer.

Notice that these solutions are equivalent to the solutions obtained from cos(2x) = 0 in Method 1.

Therefore, the combined solution set remains the same: x = (2n + 1)π/4 and x = nπ, where n is an integer.

Graphical Representation

Visualizing the solutions graphically can enhance our understanding. Plotting the functions y = cos(2x) and y = -1/sin(x) (since we are solving for when the product is zero, either function could be zero, but to visualize, we can rewrite it this way) will show the points where the graphs intersect the x-axis or intersect each other, representing the solutions to the equation. The intersection points represent the values of x where cos(2x)sin(x) = 0. The periodic nature of these functions is clearly visible in the graph, demonstrating the repetitive nature of the solutions.

Explanation of the Periodic Nature of Solutions

The periodic nature of the solutions stems directly from the periodic nature of the sine and cosine functions. Both sin(x) and cos(x) have a period of 2π. The double angle in cos(2x) affects the frequency but not the fundamental periodic behavior. This means the solutions repeat themselves every 2π units along the x-axis. The general solutions we derived incorporate this periodicity by including the 'nπ' or '2nπ' terms, where 'n' can be any integer, positive or negative.

Frequently Asked Questions (FAQ)

• Q: Are there any other methods to solve this equation?

  A: While the methods described above are the most straightforward, more advanced techniques involving complex numbers and other trigonometric identities could also be employed, but they might be unnecessarily complex for this particular problem.*

• Q: How can I verify my solutions?

  A: Substitute your solutions back into the original equation, cos(2x)sin(x) = 0. If the equation holds true, your solution is correct. For example, if x = π/4, cos(2(π/4))sin(π/4) = cos(π/2)sin(π/4) = 0 * (√2/2) = 0, confirming the solution's validity.*

• Q: What is the significance of the integer 'n' in the general solution?

  A: The integer 'n' represents the number of periods of the functions. It allows us to generate all possible solutions, not just those within a single period (0 to 2π). By changing the value of 'n', you can find solutions for any interval.*

• Q: Can this equation be solved graphically?

  A: Yes, the equation can be solved graphically by plotting the function y = cos(2x)sin(x) and identifying the x-intercepts (where y = 0). These x-intercepts represent the solutions to the equation.*

Conclusion

Solving the equation cos(2x)sin(x) = 0 involves a combination of understanding the zero product property, trigonometric identities, and the periodic nature of trigonometric functions. By employing either the direct application of the zero product property or utilizing trigonometric identities, we arrived at the same general solution set. Remember that understanding the underlying principles, not just memorizing the steps, is key to mastering trigonometric equations and solving similar problems in the future. This equation serves as a strong foundation for tackling more complex trigonometric problems. The graphical representation and the explanation of the periodic solutions further solidify our understanding. This comprehensive guide should equip you with the knowledge and tools to confidently solve such equations and appreciate the elegance and power of trigonometry.