State the order of the following ordinary differential equations and state if they are (i) time- invariant or not, and (ii) linear or not (a) x + xexx + x = 0 (b) etx + x + x = 0 (c) x + sin (tx) x + x = 0 Determine if the following differential equations are linear or not by checking if they satisfy the super-position principle (a) x + tx = 0 (b) x + xt = 0