1. Determine which of the following equations are differential: a) $y^{\prime}+3 x=0$; b) $y^{2}+x^{2}=5$; c) $y=e^{x}$; d) $y^{\prime} y-x=0$; e) $y=\ln |x|+C$; f) $2 d y+3 x d x=0$.
Solution. Equations b), c), d) are not differential, as they do not contain the derivative of the unknown function or differentials of the argument and the unknown function; equations a), e), f) are differential.