Answer:
The least cost of fencing for the rancher is $1200
Step-by-step explanation:
Let <em>x</em> be the width and <em>y </em>the length of the rectangular field.
Let <em>C </em>the total cost of the rectangular field.
The side made of heavy duty material of length of <em>x </em>costs 16 dollars a yard. The three sides not made of heavy duty material cost $4 per yard, their side lengths are <em>x, y, y</em>. Thus
![C=4x+4y+4y+16x\\C=20x+8y](https://tex.z-dn.net/?f=C%3D4x%2B4y%2B4y%2B16x%5C%5CC%3D20x%2B8y)
We know that the total area of rectangular field should be 2250 square yards,
![x\cdot y=2250](https://tex.z-dn.net/?f=x%5Ccdot%20y%3D2250)
We can say that ![y=\frac{2250}{x}](https://tex.z-dn.net/?f=y%3D%5Cfrac%7B2250%7D%7Bx%7D)
Substituting into the total cost of the rectangular field, we get
![C=20x+8(\frac{2250}{x})\\\\C=20x+\frac{18000}{x}](https://tex.z-dn.net/?f=C%3D20x%2B8%28%5Cfrac%7B2250%7D%7Bx%7D%29%5C%5C%5C%5CC%3D20x%2B%5Cfrac%7B18000%7D%7Bx%7D)
We have to figure out where the function is increasing and decreasing. Differentiating,
![\frac{d}{dx}C=\frac{d}{dx}\left(20x+\frac{18000}{x}\right)\\\\C'=20-\frac{18000}{x^2}](https://tex.z-dn.net/?f=%5Cfrac%7Bd%7D%7Bdx%7DC%3D%5Cfrac%7Bd%7D%7Bdx%7D%5Cleft%2820x%2B%5Cfrac%7B18000%7D%7Bx%7D%5Cright%29%5C%5C%5C%5CC%27%3D20-%5Cfrac%7B18000%7D%7Bx%5E2%7D)
Next, we find the critical points of the derivative
![20-\frac{18000}{x^2}=0\\\\20x^2-\frac{18000}{x^2}x^2=0\cdot \:x^2\\\\20x^2-18000=0\\\\20x^2-18000+18000=0+18000\\\\20x^2=18000\\\\\frac{20x^2}{20}=\frac{18000}{20}\\\\x^2=900\\\\\mathrm{For\:}x^2=f\left(a\right)\mathrm{\:the\:solutions\:are\:}x=\sqrt{f\left(a\right)},\:\:-\sqrt{f\left(a\right)}\\\\x=\sqrt{900},\:x=-\sqrt{900}\\\\x=30,\:x=-30](https://tex.z-dn.net/?f=20-%5Cfrac%7B18000%7D%7Bx%5E2%7D%3D0%5C%5C%5C%5C20x%5E2-%5Cfrac%7B18000%7D%7Bx%5E2%7Dx%5E2%3D0%5Ccdot%20%5C%3Ax%5E2%5C%5C%5C%5C20x%5E2-18000%3D0%5C%5C%5C%5C20x%5E2-18000%2B18000%3D0%2B18000%5C%5C%5C%5C20x%5E2%3D18000%5C%5C%5C%5C%5Cfrac%7B20x%5E2%7D%7B20%7D%3D%5Cfrac%7B18000%7D%7B20%7D%5C%5C%5C%5Cx%5E2%3D900%5C%5C%5C%5C%5Cmathrm%7BFor%5C%3A%7Dx%5E2%3Df%5Cleft%28a%5Cright%29%5Cmathrm%7B%5C%3Athe%5C%3Asolutions%5C%3Aare%5C%3A%7Dx%3D%5Csqrt%7Bf%5Cleft%28a%5Cright%29%7D%2C%5C%3A%5C%3A-%5Csqrt%7Bf%5Cleft%28a%5Cright%29%7D%5C%5C%5C%5Cx%3D%5Csqrt%7B900%7D%2C%5C%3Ax%3D-%5Csqrt%7B900%7D%5C%5C%5C%5Cx%3D30%2C%5C%3Ax%3D-30)
Because the length is always positive the only point we take is
. We thus test the intervals
and ![(30, \infty)](https://tex.z-dn.net/?f=%2830%2C%20%5Cinfty%29)
![C'(20)=20-\frac{18000}{20^2} = -25 < 0\\\\C'(40)= 20-\frac{18000}{20^2} = 8.75 >0](https://tex.z-dn.net/?f=C%27%2820%29%3D20-%5Cfrac%7B18000%7D%7B20%5E2%7D%20%3D%20-25%20%3C%200%5C%5C%5C%5CC%27%2840%29%3D%2020-%5Cfrac%7B18000%7D%7B20%5E2%7D%20%3D%208.75%20%3E0)
we see that total cost function is decreasing on
and increasing on
. Therefore, the minimum is attained at
, so the minimal cost is
![C(30)=20(30)+\frac{18000}{30}\\C(30)=1200](https://tex.z-dn.net/?f=C%2830%29%3D20%2830%29%2B%5Cfrac%7B18000%7D%7B30%7D%5C%5CC%2830%29%3D1200)
The least cost of fencing for the rancher is $1200
Here’s the diagram: