Issue in as 6 & as 10

In the AS 6 on Depreciation Accounting,

the depreciation rate under WDV is calculated as under:

1 - (10000/100000) ^ 1/5

where,

scrap =10000

cost of asset = 100000

life = 5 years.

My question is how to solve the above equation?

Secondly,  if the Cost of an asset is 10L inclusive of  excise duty 2L.

What is the depreciable value of this asset if

1. Cenvat can be availed for the excise duty paid.

2. No cenvat is available.

3. 20% of excise duty can be claimed as cenvat.

thanks.