If the straight-line depreciation method is used, the annual average investment amount used in calculating rate of return is calculated as (beginning book value + ending book value)/2.

This is correct according to the straight-line depreciation method, which assumes that the value of an asset decreases equally over its lifespan. Therefore, the average investment amount would be calculated as the beginning book value (cost of the asset) plus the ending book value (residual value) divided by 2.