What technique is primarily used for integrating functions containing square roots?

The correct choice for integrating functions containing square roots is trigonometric substitution. This method is particularly effective when dealing with square roots of quadratic expressions, such as (\sqrt{a^2 - x^2}), (\sqrt{x^2 - a^2}), and (\sqrt{x^2 + a^2}).

Trigonometric substitution utilizes the relationships between trigonometric functions and right triangles, allowing for simplification of the integrals. For instance, if you encounter an integral involving (\sqrt{a^2 - x^2}), you can use the substitution (x = a \sin(\theta)), which transforms the square root into a simpler expression involving (a) and (\theta).

This technique not only simplifies the integrand but also provides a clear method to evaluate the integral that can be expressed back in terms of (x) after integration.

While integration by substitution is sometimes useful, it is generally applicable to functions that can be transformed into simpler forms, but it is not specifically optimal for the square root scenarios that trigonometric substitution directly addresses. Other methods like integration by parts and partial fractions have their relevance, but they are not the primary techniques recommended for functions