Find the smallest positive solution to tan(2x) + tan(3x) = sec(3x) in radians.

1 answer

To solve the equation

\[ \tan(2x) + \tan(3x) = \sec(3x), \]

we start by rewriting the equation in terms of sines and cosines. Recall that:

  • \(\tan(x) = \frac{\sin(x)}{\cos(x)}\)
  • \(\sec(x) = \frac{1}{\cos(x)}\)

Thus, the given equation can be expressed as:

\[ \frac{\sin(2x)}{\cos(2x)} + \frac{\sin(3x)}{\cos(3x)} = \frac{1}{\cos(3x)}. \]

Multiplying both sides by \(\cos(2x)\cos(3x)\):

\[ \sin(2x)\cos(3x) + \sin(3x)\cos(2x) = \cos(2x). \]

Next, apply the angle addition formula for sine:

\[ \sin(a + b) = \sin(a)\cos(b) + \cos(a)\sin(b). \]

Now we rewrite the left side:

\[ \sin(2x)\cos(3x) + \sin(3x)\cos(2x) = \sin(2x + 3x) = \sin(5x). \]

Thus, we have:

\[ \sin(5x) = \cos(2x). \]

To solve this equation, we can express \(\cos(2x)\) in terms of sine:

\[ \sin(5x) = \sin\left(\frac{\pi}{2} - 2x\right). \]

Using the sine equality, we have two cases to consider:

  1. \(5x = \frac{\pi}{2} - 2x + 2k\pi\) for some integer \(k\)
  2. \(5x = \pi - \left(\frac{\pi}{2} - 2x\right) + 2k\pi\) for some integer \(k\)

Case 1:

From \(5x = \frac{\pi}{2} - 2x + 2k\pi\):

Adding \(2x\) to both sides gives:

\[ 7x = \frac{\pi}{2} + 2k\pi \quad \Rightarrow \quad x = \frac{\pi}{14} + \frac{2k\pi}{7}. \]

Case 2:

From \(5x = \pi - \left(\frac{\pi}{2} - 2x\right) + 2k\pi\):

This simplifies to:

\[ 5x = \pi - \frac{\pi}{2} + 2x + 2k\pi \quad \Rightarrow \quad 5x = \frac{\pi}{2} + 2x + 2k\pi. \]

Subtracting \(2x\) from both sides gives:

\[ 3x = \frac{\pi}{2} + 2k\pi \quad \Rightarrow \quad x = \frac{\pi}{6} + \frac{2k\pi}{3}. \]

Finding the smallest positive solution

Now we need to evaluate the possible solutions for \(k = 0\):

  1. From Case 1: \(x = \frac{\pi}{14}\).
  2. From Case 2: \(x = \frac{\pi}{6}\).

We need to compare \(\frac{\pi}{14}\) and \(\frac{\pi}{6}\) to find the smallest positive root:

Calculating decimal approximations:

  • \(\frac{\pi}{14} \approx 0.224\)
  • \(\frac{\pi}{6} \approx 0.524\)

The smallest positive solution is thus:

\[ \boxed{\frac{\pi}{14}}. \]