Step 1 :Let the distance from the farmer's house to the point P on the river be denoted as \(x\), and the distance from point P to the farm be denoted as \(y\).
Step 2 :The farmer's house, the point P on the river, and the farm form a right triangle, with the distance from the farmer's house to the farm as the hypotenuse.
Step 3 :By the Pythagorean theorem, we have: \(y^2 = (5+x)^2 + 6^2\).
Step 4 :Solving for \(y\), we get: \(y = \sqrt{(5+x)^2 + 36}\).
Step 5 :The total distance the farmer has to walk is the sum of \(x\) and \(y\), which we denote as \(D\): \(D = x + y\).
Step 6 :Substituting \(y\) into the equation for \(D\), we get: \(D = x + \sqrt{(5+x)^2 + 36}\).
Step 7 :To find the minimum distance, we need to find the derivative of \(D\) with respect to \(x\), set it equal to zero, and solve for \(x\): \(D' = 1 + \frac{(5+x)}{\sqrt{(5+x)^2 + 36}}\).
Step 8 :Setting \(D'\) equal to zero, we get: \(1 + \frac{(5+x)}{\sqrt{(5+x)^2 + 36}} = 0\).
Step 9 :Solving for \(x\), we get: \(x = -1 - \sqrt{41}\).
Step 10 :However, since \(x\) represents a distance, it cannot be negative. Therefore, the minimum distance occurs when \(x = 0\).
Step 11 :Substituting \(x = 0\) into the equation for \(D\), we get: \(D = 0 + \sqrt{(5+0)^2 + 36}\).
Step 12 :Simplifying, we get: \(D = \sqrt{25 + 36}\).
Step 13 :Further simplifying, we get: \(D = \sqrt{61}\).
Step 14 :Therefore, the minimum distance the farmer has to walk is \(\sqrt{61}\) miles, or approximately 7.81 miles.
Step 15 :So, the final answer is \(\boxed{\sqrt{61}}\) miles.