√- 81 doesn't result in a rational number because you can't get a negative from squaring any number. Instead you get a number with an imaginary number.
13 - √- 81 = 13 - 9i
we are given

where
F is the number of board feet
d is the diameter (in inches) of the log
Since, we have to minimize F
so, firstly we will find it's derivative


now, we can set it to 0
and then we can solve for d


so, we get diameter as
..............Answer
What are the units? Tell me the units so I can solve this please.
Hi if you showed a picture of the circle graph I would be happy to answer your question.