题目为 A company purchased equipment for $50,000 on 1 January 2009. It is depreciating the equipment over a period of 10 years on a straight-line basis for accounting purposes, but for tax purposes, it is using the declining balance method at a rate of 20%. Given a tax rate of 30%, the deferred tax liability as at the end of 2011 is closest to:A. $420.
B. $2,820.
C. $6,720.   

答案是
B is correct. The deferred tax liability is equal to the tax rate times the difference between the carrying amount of the asset and the tax base.
Value for accounting purposes after 3 years: 50,000 – [3 x (50,000 ÷ 10)]= $35,000
Value for tax purposes:  Carrying amount = Start of year balance × (1 – 0.20)
After three years:   50,000 × 0.8 × 0.8 × 0.8 =   25,600
Difference between accounting and tax values  9,400
Deferred tax liability @ 30%: 30% × 9,400 = 2,820

我想问一下为什么不能算累计的DTL 而只算2011年当年的DTL呢？