Mathematics

Mathematics

Problem

Evaluate ∫ln⁡(5x) dx\displaystyle\int \ln(5x)\,dx for x>0x>0. Which of the following is an antiderivative of ln⁡(5x)\ln(5x)? a) xln⁡(5x)−x5+Cx\ln(5x)-\dfrac{x}{5}+C b) xln⁡(5x)−x+Cx\ln(5x)-x + C c) (ln⁡(5x))22+C\dfrac{(\ln(5x))^{2}}{2}+C d) xln⁡(5x)+Cx\ln(5x)+C
Intuitive explanation

The key is recognizing that ln⁡(5x)=ln⁡(5)+ln⁡(x)\ln(5x) = \ln(5) + \ln(x), which splits this into a constant plus a familiar logarithm. You already know how to integrate ln⁡(x)\ln(x) using integration by parts (where u=ln⁡(x)u = \ln(x) and dv=dxdv = dx), and ln⁡(5)\ln(5) is just a constant multiplier. This decomposition transforms an unfamiliar integral into one you can handle with a standard technique.

Intuitive explanation

The key is recognizing that ln⁡(5x)=ln⁡(5)+ln⁡(x)\ln(5x) = \ln(5) + \ln(x), which splits this into a constant plus a familiar logarithm. You already know how to integrate ln⁡(x)\ln(x) using integration by parts (where u=ln⁡(x)u = \ln(x) and dv=dxdv = dx), and ln⁡(5)\ln(5) is just a constant multiplier. This decomposition transforms an unfamiliar integral into one you can handle with a standard technique.

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