According to the trigonometric relationship, the calculated shadow length on the soil slope on the flat land = COS 30 * 8 = 4 times the root number 3.

Total shadow length =4√3+20

According to a similar proportion: shadow length: rod length = shadow length: rod length.

Therefore, extremely long = extremely long/shadow length * shadow length =1/2 * (4 √ 3+20) = 2 √ 3+10.