# A calculus of the absurd

#### 14.3 Substitution

Linear differential equations 9595 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" 9696 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.