A patent was purchased on 1/1/2015 for 120,000 with 10 years useful life .On 1/1/2016 the company paid legal defense fees of 63,000.on 1/1/2017 the management decided to impair the patent .they expected the future net cash flow 70,000 and fair value 60,000 required calculate the amortization expense for the years : 2015,2016,2017,2018