# Linearity in Differential Equations

## Homework Statement

Is the following differential equation linear:

$yy' + 2 = 0$

## The Attempt at a Solution

I have the definition of linear as being $a_0 (t) y^{(n)} + a_1(t) y^{n-1} + a_2 (t) y^{n-2} ... = 0$. Now, presumably y is a function of t. Thus, I could define $y = a_0 (t)$ and let n=1. Thus I would satisfy my definition of linearity in differential equations. Thus, the differential equation is linear.

Is it not so?

No, $a_0(t)$ has to be a known function of t, not y or anything involving y. This equation is non-linear because you have a product of things "involving" the dependent variable y.