The first-order differential equations we have solved in the previous sections have all been either separable or linear. But many applications involve differential equations that are neither separable nor linear. In this section we illustrate (mainly with examples) substitution methods that sometimes can be used to transform a given differential equation into one that we already know how to solve.
For instance, the differential equation
with dependent variable y and independent variable x, may contain a conspicuous combination
of x and y that suggests itself as a new independent variable v. Thus the differential equation
practically demands the substitution ...