1-6 下列各式是描述系统的微分方程,其中c(t)是输出量,r(t)是输入量,试判断哪些-|||-是线性的,哪些是非线性的,哪些是变系数的?-|||-(1) dfrac ({d)^2c(t)}(d{t)^2}=c(t)dfrac (dc(t))(dt)-|||-(2) ^2dfrac ({d)^3c(t)}(d{t)^3}-(e)^-1dfrac (dc(t))(dt)+2c(t)=r(t)-|||-(3) dfrac (1)(c(t))dfrac ({d)^2c(t)}(d{t)^2}+dfrac (1)(c(t))dfrac (dc(t))(dt)+1=0-|||-(4) dfrac ({d)^2c(t)}(d{t)^2}+dfrac (1)(c(t))dfrac (dc(t))(dt)-3=0-|||-(5) (1-(t)^2)dfrac ({d)^2c(t)}(d{t)^2}-2tdfrac (dc(t))(dt)+n(n+1)c(t)=0 (n是常数)-|||-(6) dfrac ({d)^2c(t)}(d{t)^2}+dfrac (1)(t)dfrac (dc(t))(dt)+(1-dfrac ({n)^2}({t)^2})c(t)=0 (n是常数)