6.15. For each of the following second-order differential equations for causal and sta- ble LTI systems, determine whether the corresponding impulse response is under- damped, overdamped, or critically damped: d²y(1) 4dy(1)

+ 4y(t) = x(t) (a) (b) 5x() +4dy(n) + 5y(1) 7x(t) dt (c) + 20+ y(t) = x(t) d²y(1) drª (d) 5²(+4dyr) + 5y(t) +49 di = 7x(t) + di