# 13.3 Substitution

Linear differential equations ^{2}^{2}
2
The ones in the form where they can be
solved using an integrating factor are easy to solve, which is nice. Some
equations are less easy to solve, but in the same way that there are
"hidden quadratics" ^{3}^{3}
3
Have another look at the algebra section if you’re
not sure about what hidden quadratics are. we can also have
"hidden first-order linear differential equations" (which is much more of a
mouthful). By making a substitution we can rewrite many equations which don’t
look linear at first sight as linear differential equations.