Math Problem Statement

Solve 2cos^2(x) - 5cos(x) + 2 = 0 for all solutions 0 ≤ x < 2π.

Solution

The problem asks us to solve the trigonometric equation:

2cos2(x)5cos(x)+2=0,for 0x<2π.2 \cos^2(x) - 5 \cos(x) + 2 = 0, \quad \text{for } 0 \leq x < 2\pi.

Step-by-step solution:

  1. Rewrite the equation:
    Let y=cos(x)y = \cos(x). The equation becomes: 2y25y+2=0.2y^2 - 5y + 2 = 0.

  2. Solve the quadratic equation:
    Using the quadratic formula: y=b±b24ac2a,y = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}, where a=2a = 2, b=5b = -5, and c=2c = 2.

    Substitute these values: y=(5)±(5)24(2)(2)2(2)=5±25164.y = \frac{-(-5) \pm \sqrt{(-5)^2 - 4(2)(2)}}{2(2)} = \frac{5 \pm \sqrt{25 - 16}}{4}. y=5±94=5±34.y = \frac{5 \pm \sqrt{9}}{4} = \frac{5 \pm 3}{4}.

    This gives two solutions for yy: y=5+34=2(not valid, since 1cos(x)1),y = \frac{5 + 3}{4} = 2 \quad \text{(not valid, since } -1 \leq \cos(x) \leq 1\text{)}, and y=534=24=0.5.y = \frac{5 - 3}{4} = \frac{2}{4} = 0.5.

    So, cos(x)=0.5\cos(x) = 0.5.

  3. Solve for xx: Recall the general solution for cos(x)=0.5\cos(x) = 0.5 in the interval 0x<2π0 \leq x < 2\pi: x=cos1(0.5).x = \cos^{-1}(0.5). The solutions are: x=π3,x=2ππ3=5π3.x = \frac{\pi}{3}, \quad x = 2\pi - \frac{\pi}{3} = \frac{5\pi}{3}.

  4. Write the final solution: The solutions to the equation are: x=π3,5π3.x = \frac{\pi}{3}, \frac{5\pi}{3}.

    In decimal form (to two decimal places): x1.05,5.24.x \approx 1.05, 5.24.

Final Answer:

x=1.05,5.24x = 1.05, 5.24

Would you like a detailed explanation of any step or have further questions?


Here are five related questions to deepen your understanding:

  1. What is the domain and range of cos(x)\cos(x)?
  2. How can you verify that cos(x)=2\cos(x) = 2 is invalid?
  3. How does the quadratic formula work in trigonometric contexts?
  4. What are other methods to solve quadratic trigonometric equations?
  5. Can you graph y=2cos2(x)5cos(x)+2y = 2\cos^2(x) - 5\cos(x) + 2 to visualize the solution?

Tip: Always check the interval restrictions when solving trigonometric equations!

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Math Problem Analysis

Mathematical Concepts

Trigonometry
Quadratic Equations
Cosine Function

Formulas

Quadratic formula: y = (-b ± √(b^2 - 4ac)) / 2a
Cosine identity: cos^2(x) + sin^2(x) = 1

Theorems

Quadratic Formula
Properties of Trigonometric Functions

Suitable Grade Level

Grades 10-12