Integrating factor of the differential equation
\[
\frac{dy}{dx} + y = \frac{x^3 + y}{x}
\]
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For first-order linear differential equations, the integrating factor is calculated using the formula \( I.F. = e^{\int P(x) \, dx} \), where \( P(x) \) is the coefficient of \( y \) in the equation.