SOLUTION: Please help me with this problem.I don't understand it. My teacher never went over these kinds of problems in class. I would really appreciate it if you would explain and answer.

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Question 697679: Please help me with this problem.I don't understand it. My teacher never went over these kinds of problems in class. I would really appreciate it if you would explain and answer.
A farmer has 526 meters of fencing available to enclose a rectangular portion of his land. One side of the rectangle being fenced lies along a river, so only three sides require fencing.
(a) Express the area A of the rectangle as a function of x, where x is the length of the side parallel to the river.
(b) For what value of x is the area largest?
Thanks in advance!

Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
+x+ is the length of the side parallel to the river.
Each of the sides that are perpendicular to the river
are +%28+526+-+x+%29+%2F+2+
Let +A+ = area of enclosed portion
+A+=+x%2A%28+526+-+x+%29+%2F+2+
+A+=+%28+-x%5E2+%2B+526x+%29+%2F+2+
+A+=+%28-1%2F2%29%2Ax%5E2+%2B+263x+
This is a parabola which has a maximum because the
coefficient of the +x%5E2+ term is negative and the +x+
term is positive. The maximum is at x%5Bmax%5D+=++-b%2F%282a%29+
when the equation has the form +A+=+ax%5E2+%2B+bx+%2B+c+
+a+=+-1%2F2+
+b+=+263+
+-b%2F%282a%29+=+-263+%2F+%28+2%2A%28-1%2F2%29+%29+
+-b%2F%282a%29+=+263+
263 meters answer
This is the x-co-ordinate of the maximum area, which is the
length of fencing parallel to the river.
-------------
If you want the maximum Area, just plug this back into the equation
+A%5Bmax%5D+=+%28-1%2F2%29%2A263%5E2+%2B+263%2A263+
+A%5Bmax%5D+=+-69169%2F2+%2B+69169+
+A%5Bmax%5D+=+34584.5+ m2
------------------------
It's easy to test this result. if +x+ is slightly more than or less
than +263+, the area should go down slightly also
If I say +x+=+262.8+, then
+A+=+%28-1%2F2%29%2A262.8%5E2+%2B+262.8%2A263+
+A+=+%28-1%2F2%29%2A69063.84+%2B+69116.4+
+A+=+-34531.92+%2B+69116.4+
+A+=+34584.48+ It went down
You can check for +x+=+263.2+
The area should also go down by roughly
the same amount.