1. (2+2+2-6pts) Determine the order, dependent and independent variables of the given differential equation and state whether the equation is linear or nonlinear. (a) t. dy + 2ety = 3t2 + 1 dt order: dep. var.: indep. var.: linearon-linear (circle one) (b) u" + 6u' - u' = r order: dep. var.: indep. var.: linearon-linear (circle one) (c) x" + (siny)x' = yx + 5 order: dep. var.: indep. var.: linearon-linear (circle one)