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Posted by **gabriel** on Friday, January 18, 2013 at 6:29pm.

how can i come up with a quadratic equation from this problem????

- MATH 11 help PLease! -
**Damon**, Friday, January 18, 2013 at 6:41pmSketch the problem

You need 5 lengths of fence of length w and 1 length e for end

area = A = w e

5 w + e = 90

5 w + A/w = 90

5 w^2 - 90 w + A = 0

- MATH 11 help PLease! -
**gabriel**, Friday, January 18, 2013 at 6:45pmwhere did you get 5?

- MATH 11 help PLease! -
**Damon**, Friday, January 18, 2013 at 6:48pmNow if you do not know calculus, find the vertex of the parabola by completing the square

5 w^2 - 90 w = -A

w^2 - 18 w = -A/5

w^2 -18 w + 81 = -A/5 + 81

(w-9)^2 = -(1/5)(A - 405)

so max at w = 9 , A = 405

then

5 (9) + e = 90

45 + e = 90

e = 45

so 45 by 9

- MATH 11 help PLease! -
**gabriel**, Friday, January 18, 2013 at 6:52pmi know how to get the vertex and all the other stuff,

i just don't understand where did you get 5?

- MATH 11 help PLease! -
**Damon**, Friday, January 18, 2013 at 6:56pmTo get 5, DRAW A Picture

9 foot fence at 0, 11.25 , 22.5 , 33.75 , 45

and a 45 foot fence across the end

- MATH 11 help PLease! -
**Damon**, Friday, January 18, 2013 at 6:56pmfour fence sections requires five posts :)

- MATH 11 help PLease! -
**Damon**, Friday, January 18, 2013 at 6:57pmbecause you need one at the start, zero.

- MATH 11 help PLease! -
**Damon**, Friday, January 18, 2013 at 6:59pm(unless you close the figure topologically by joining the last section to the first.)

- MATH 11 help PLease! -
**gabriel**, Friday, January 18, 2013 at 7:00pmaah ok.. thanks a lot!!!;)

- MATH 11 help PLease! -
**Damon**, Friday, January 18, 2013 at 7:00pm0, 11.25 , 22.5 , 33.75 , 45

notice four commas seperating five numbers

- MATH 11 help PLease! -
**Damon**, Friday, January 18, 2013 at 7:01pmI am belaboring this because it is not the last time you will have to think about it.

- MATH 11 help PLease! -
**gabriel**, Friday, January 18, 2013 at 7:05pmok,i will just try it myself so that i can understand it.. thanks..