4. Determine which of the following systems is linear or non-linear. Show proof

a. y(t) = 2x(t) – 5

b.·dy/dt + 4y(t) = 2x(t)

C.dy/dt+2ty(t) = x(t)

d.dy/dt+ 4y(t) + 1 = x(t)

e.(dy/dt) + y(t) = x²(t)

f. (dy/dt)² + y(t) = x(t)