Mechanical energy of a spring system

[Schulze]

Homework Statement

A damped mass-spring system oscillates at
285 Hz. The time constant of the system is
8.8 s. At t = 0 the amplitude of oscillation
is 1.3 cm and the energy of the oscillating
system is 36 J.
Part 1: What is the amplitude of oscillation at t =
8.7 s?
Answer in units of cm
Part 2: How much energy is dissipated in the first
period (8.7 s interval)?
Answer in units of J

Part 3: How much energy is dissipated in the second
period (8.7 s interval)?
Answer in units of J

Homework Equations

A = A(initial) * e^{-(t/time constant)}
E = E(initial) * e^{-(t/time constant)}

I followed the method of the attached picture and couldn't get the correct answer.

The Attempt at a Solution

Part 1: Answered correctly:
A(8.7 s) = (1.3 cm) * e^-(8.7/8.8) = 0.4837088518 cm

Part 2:
Change in mechanical energy between 0 and 7.8 seconds:
ΔE = -E(initial) * (e^-(8.7/8.8) - e^-(0/8.8))
ΔE = -(36 J) * (e^-(8.7/8.8) - 1)
ΔE = 22.604985 J
Which was incorrect

Part 3:
ΔE = -(36 J) * (e^-(17.4/8.8) - e^-(8.7/8.8))
ΔE = 8.4109474 J
Which was also incorrect.

 

[BvU]

Check the small print (I can hadly read it): Energy is proportional to A², so one of your equations isn't right