如果以y作为变量，自然不是线性函数了。

equation 3 and 4 are linear.
In differential equations, the term "linear" means something different. It doesn't mean that y(t) is a linear function. It means that no term in the equation has a power more than 1. Sin and Cos are not linear either since they are not power=1 functions either.

没看懂2楼的答案