6) Find period, radian frequency, and natural frequency, also sketch the functions (approximately): a) x(t)= 4sin(6πt+90), b) y(t)=2cos(8t-90), c) z(t)=4e-0.2 cos(4t)u(t) d) w(t)=3tcos(4t)u(t) e) d(t)=2te-0.2 cos(4t)u(t)