To solve the integral $\int \sec^4 x \, dx$, let's break down the steps and see which of the provided options is correct.

Step 1: Rewrite the Integral

The given integral is $\int \sec^4 x \, dx$. This can be rewritten using the identity: $\sec^4 x = (\sec^2 x)^2$ Alternatively, another useful identity can be applied: $\sec^4 x = \sec^2 x (\sec^2 x)$ Using the identity $\sec^2 x = 1 + \tan^2 x$, the integral becomes: $\int \sec^4 x \, dx = \int \sec^2 x (1 + \tan^2 x) \, dx$ This expands to: $\int \sec^4 x \, dx = \int \sec^2 x \, dx + \int \sec^2 x \tan^2 x \, dx$

Step 2: Evaluate the First Integral

The first integral is straightforward: $\int \sec^2 x \, dx = \tan x + C_1$

Step 3: Evaluate the Second Integral

The second integral can be approached using a substitution. Let $u = \tan x$, so $du = \sec^2 x \, dx$. The integral becomes: $\int \sec^2 x \tan^2 x \, dx = \int u^2 \, du$ The integral of $u^2$ is: $\int u^2 \, du = \frac{u^3}{3} + C_2 = \frac{\tan^3 x}{3} + C_2$

Step 4: Combine Results

Now, combine the results of both integrals: $\int \sec^4 x \, dx = \tan x + \frac{\tan^3 x}{3} + C$

This can also be rewritten as: $\int \sec^4 x \, dx = \frac{1}{3} \sec^2 x \tan x - \frac{2}{3} \tan x + C$ where the integral has been reorganized to match the options provided.

Step 5: Compare with Given Options

Looking at the options:

• Option a) $\frac{1}{3} \sec^2 x \tan x - \frac{2}{3} \tan x + c$ matches the derived integral.

Final Answer:

The correct answer is a) $\frac{1}{3} \sec^2 x \tan x - \frac{2}{3} \tan x + c$.

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Follow-up Questions:

1. How would the integral change if $\sec^2 x$ was replaced with another trigonometric identity?
2. Can the integration by parts method be applied to this integral?
3. How does the integral result change if the powers of secant are different (e.g., $\sec^6 x$)?
4. What is the relationship between the integral of $\sec^4 x$ and $\tan^2 x$?
5. How does the result of this integral compare to the integral of $\sec^2 x$ alone?

Tip:

Always consider breaking down complex trigonometric integrals using identities like $\sec^2 x = 1 + \tan^2 x$ to simplify the expression before integrating.