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 Nov 10 awarded Popular Question Nov 9 awarded Notable Question Nov 9 awarded Popular Question Nov 7 awarded Popular Question Oct 22 awarded Notable Question Oct 19 awarded Popular Question Oct 11 awarded Notable Question Oct 7 awarded Popular Question Oct 5 awarded Good Question Sep 16 comment When is a system called linear? @HansEngler I don't see how this comment helps. I did have Linear Algebra classes and I was good in that. However, the solutions suggest that there is an implicit dependance of $x$ on $w$ or a different definition of "linear". I should probably have asked this question on engineering.stackexchange.com, but I thought it was something specific to differential equations. Or a well known use case of differential equations. I assumed that because there is no explanation in any of the material we were given in this lecture. Sep 15 revised When is a system called linear? deleted 82 characters in body Sep 15 asked When is a system called linear? Aug 31 awarded Announcer Aug 23 awarded Popular Question Jul 1 awarded Nice Question May 28 awarded Announcer May 15 accepted Why does $a\cdot \cos(kx)+b\cdot\sin(kx)=\sqrt{a^2+b^2}\sin(kx+\phi)$ hold? May 15 comment Why does $a\cdot \cos(kx)+b\cdot\sin(kx)=\sqrt{a^2+b^2}\sin(kx+\phi)$ hold? @KittyL Yes, that seems to be the correct answer as the title of the section is "amplitude spectrum" (and I have never seen a negative amplitude). I am not sure what I should do now with this question. Delete it? Do you want to post your comment as an answer? May 15 reviewed Approve Why does $a\cdot \cos(kx)+b\cdot\sin(kx)=\sqrt{a^2+b^2}\sin(kx+\phi)$ hold? May 15 asked Why does $a\cdot \cos(kx)+b\cdot\sin(kx)=\sqrt{a^2+b^2}\sin(kx+\phi)$ hold?