Hwan also wants to determine the sum$-$of$-$years depreciation balance for the first year and the last year of the useful life of the construction equipment. Determine these amounts as follows:

a$.$ For the yearly allowance for the first year, use a function to calculate the sum$-$of$-$years depreciation balance, using a cell reference to Year $1$ of the declining balance section as the current period.

b$.$ For the yearly allowance for the last year, use a function to calculate the sum$-$of$-$years depreciation balance, using a cell reference to Year $10$ of the declining balance as the current period.