# Homework Help: Is the system linear or nonlinear

1. Sep 8, 2016

### physicsgirl199

1. The problem statement, all variables and given/known data
3y(t)+2=x(t)

2. Relevant equations

k1y1(t) + k2y2(t) + 2(k1+k2) = k1x1(t)+k2x2(t)

3. The attempt at a solution

I know the system is non linear but I cannot explain why. It has something to do with 2(k1+k2) but I am unsure.

2. Sep 8, 2016

### Staff: Mentor

Why do you say you know it's non-linear?

Last edited: Sep 8, 2016
3. Sep 8, 2016

### Staff: Mentor

How are you defining the term "linear"? In some contexts, this term implies that the graph of the relationship is a straight line. In the context of linear transformations, the conditions for linearity are that $L(x_1 + x_2) = L(x_1) + L(x_2)$ and that $L(cx) = cL(x)$.

You put this equation in the Relevant equations section. How is it relevant to this problem?

4. Sep 9, 2016

### physicsgirl199

it is both the properties you have described but I have combined them into one equation.

also it was marked as nonlinear in the solution manual

5. Sep 9, 2016

### Staff: Mentor

You could write the given equation as $y(t) = \frac 1 3 x(t) - \frac 2 3$
Now check the two properties separately.
1) Is $y(t_1 + t_2) = y(t_1) + y(t_2)$?
2) Is $y(k \cdot t_1) = k \cdot y(t_1)$?

If both of the above are true for all values of t, the relationship is linear; otherwise, it's nonlinear.

6. Sep 9, 2016

### Ray Vickson

I think it is a bit ambiguous.

If your equation $3y(t)+2 = x(t)$ describes a "dynamical system", then it is not linear because if $(x_1(t),y_(t))$ and $(x_2(t),y_2(t))$ are two solutions, the pair $(x_1(t)+x_2(t),y_1(t)+y_2(t))$ is not a solution, nor is $(cx_1(t),cy_2(t))$ for a constant $c \neq 1$.

On the other hand the equation $x - 3y = 2$ is what would normally be called "linear equation"---meaning that the left-hand-side in $f(x,y) = c$ is a linear function of $x,y$. Perhaps that is a bit of abuse of language, but it is nevertheless standard usage in describing equations. We do something similar when we call a differential equation such as $dy/dx + 2 y = x^2$ a (non-homogeneous) linear differential equation!