How do you determine if a system is linear or nonlinear?

What defines a linear system and a nonlinear system? What should I look for?

the squared term in the top coordinate is a dead give-away.

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Originally Posted by Deveno

the squared term in the top coordinate is a dead give-away.

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Does that make it linear? My book doesn't define what a linear and nonlinear system is so I'm struggling to find the differences.

That squared term makes it non linear. Linear... like a line y= mx + b.

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Originally Posted by Scopur

That squared term makes it non linear. Linear... like a line y= mx + b.

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Is that it? Is it just a reference to the graph that comes out from the equations?
Well that's easy. I can't believe I didn't get that, I thought there was more to it lol, thanks