What function has derivative 2 x {\displaystyle {\frac {2}{x}}} ? What function has derivative x 4 4 {\displaystyle {\frac {x^{4}}{4}}} ? Think about the sum rule for differentiation. What does this tell us about a function F {\displaystyle F} which has derivative F ′ ( x ) = 2 x + x 4 4 {\displaystyle F^{'}(x)={\frac {2}{x}}+{\frac {x^{4}}{4}}} .
