When faced with the integral of a product of two functions, such as \( \int 6x e^{x^2} \, dx \), a common approach is to use variable substitution. However, if the problem changes to \( \int 6x e^x \, dx \), this method becomes ineffective. In such cases, integration by parts (IBP) is a valuable technique to apply.
Integration by parts is based on the product rule of differentiation, which states that the derivative of the product of two functions \( f(x) \) and \( g(x) \) can be expressed as:
\[\frac{d}{dx}(f(x)g(x)) = f'(x)g(x) + f(x)g'(x)\]
To derive the integration by parts formula, we can integrate both sides with respect to \( x \), leading to:
\[\int f'(x)g(x) \, dx = f(x)g(x) - \int f(x)g'(x) \, dx\]
Rearranging this gives us the integration by parts formula:
\[\int u \, dv = uv - \int v \, du\]
In this formula, \( u \) is a function that simplifies when differentiated, and \( dv \) is a function that can be easily integrated to find \( v \).
To apply this to our example \( \int 6x e^x \, dx \), we can choose:
- Let \( u = 6x \) (which simplifies to \( du = 6 \, dx \))
- Let \( dv = e^x \, dx \) (which integrates to \( v = e^x \))
Substituting these into the integration by parts formula gives:
\[\int 6x e^x \, dx = 6x e^x - \int e^x \cdot 6 \, dx\]
This simplifies to:
\[6x e^x - 6 \int e^x \, dx\]
Since \( \int e^x \, dx = e^x + C \), we can substitute this back into our equation:
\[6x e^x - 6e^x + C\]
Thus, the final result for the integral \( \int 6x e^x \, dx \) is:
\[6x e^x - 6e^x + C\]
Understanding how to identify \( u \) and \( dv \) is crucial for successfully applying integration by parts. Practice with various examples will help solidify this technique, making it easier to tackle more complex integrals in the future.