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Calc Optimization Help

Blogs > FragKrag
Post a Reply
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FragKrag
Profile Blog Joined September 2007
United States11552 Posts
Last Edited: 2009-01-21 03:34:15
January 21 2009 01:31 GMT
#1
so I'm having trouble with optimization I don't think I understand it well enough. I know the whole two formula plugin and derive, but I don't think I'm getting it.

You have 20 ft of wire, and you need to make an equilateral triangle and a square. How much should be used for each figure so that the area is maximized?

x for square, b for triangle
so far, I have:
4x+3b=20
x^2+1/2(bh)=A
h=root(b^2-[b/2]^2)

x=5-3/4(b)

=> 25-30/4(b)+9/16(b^2)+b/2(root[b^2-(b/2)^2])

it seems like it's way too complicated.. Am I doing it wrong?

might be more questions later ;;


ok, problem 2:

Pink Pig is in the muddy field 4 mi from the nearest point, P, on Farm Road 5. She wants to get to Sally's Slop House on that road, 7 mi to the right of point P in the shortest possible time. Since her maximum possible speeds are 3 mph in mud and 6 mph on the road, she mentally calculates that she must maintain maximum possible speed and intersect the road ___ mi to the right of point P.

*
*TL CJ Entusman #40* "like scissors does anything to paper except MAKE IT MORE NUMEROUS" -paper
mahnini
Profile Blog Joined October 2005
United States6862 Posts
January 21 2009 01:32 GMT
#2
1a2a3a HAHAHAHAHAHHAA
the world's a playground. you know that when you're a kid, but somewhere along the way everyone forgets it.
clazziquai
Profile Blog Joined October 2007
6685 Posts
January 21 2009 01:32 GMT
#3
oh god, i sucked at calculus, and i hated this section the most lol.
#1 Sea.Really Fan / #1 Nesh Fan / Terran Forever~
EsX_Raptor
Profile Blog Joined February 2008
United States2801 Posts
January 21 2009 01:33 GMT
#4
Find the derivative of the formulas and solve for X when f(x) = 0
FragKrag
Profile Blog Joined September 2007
United States11552 Posts
January 21 2009 01:35 GMT
#5
yeah, but taking the derivative of the second part is going to be a mess t_t and the book usually points out an easier way. Just wanted to see if there is a smarter way, or if I just have to brute force this shit.
*TL CJ Entusman #40* "like scissors does anything to paper except MAKE IT MORE NUMEROUS" -paper
Ecael
Profile Joined February 2008
United States6703 Posts
January 21 2009 01:39 GMT
#6
That's the most they can throw at you for Calc 1, annoying derivatives, suck it up and doing it is probably the only way.

Though that kind of derivative looks pretty normal for something from optimization.
FragKrag
Profile Blog Joined September 2007
United States11552 Posts
January 21 2009 01:41 GMT
#7
auuuuuugghhhh i hate algebra zzzzz

thanks for the help though : )
*TL CJ Entusman #40* "like scissors does anything to paper except MAKE IT MORE NUMEROUS" -paper
EsX_Raptor
Profile Blog Joined February 2008
United States2801 Posts
January 21 2009 01:43 GMT
#8
if you're in a hurry you can always use those online derivative calculators... but i'd recommend you use those as a ways to check your work, for I assume your teacher won't allow you to use the internet while taking the test
micronesia
Profile Blog Joined July 2006
United States24664 Posts
January 21 2009 01:49 GMT
#9
I just slapped something together and the calc was trivial compared to the other stuff:

For starters I assumed the length of the string is L, the perimeter of the square is p1, the perimeter of the triangle is p2, and p1+p2=L.

Area1 = (p1 / 4)^2 = p1 / 16
Area2 = .5 * (p2 / 3) * h where I used the law of sines to get h = p2*sqrt(3)/6

So A = A1+A2
Take the derivative of the quadratic (easy if you set i up decently) to get a linear equation, and then set it equal to zero to see if there is a max/min/etc...

And I get that the triangle should be 20/(1+sqrt(3)/2) and thus the square should be 20(1-1/(1+sqrt(3)/2))
ModeratorThere are animal crackers for people and there are people crackers for animals.
Kentor *
Profile Blog Joined December 2007
United States5784 Posts
Last Edited: 2009-01-21 02:07:33
January 21 2009 01:59 GMT
#10
Let's say

s = side of square and b = side of triangle

4s + 3b = 20, or b = (20 - 4s)/3, or s = (20 - 3b)/4

For an equilateral triangle, the height, in terms of its base, is h = root(3)/2 * b
So the area of the triangle is 1/2 * b * h = root(3)/4 * b^2

Total area will be

s^2 + root(3)/4 * b^2 = A

substitute either s in terms of b, or b in terms of s, let's just choose the first one

((20 - 3b)/4)^2 + root(3)/4 * b^2 = A
(9b^2 - 120b + 400)/16 + root(3)/4 * b^2 = A
9/16 * b^2 - 15/2 b + 25 + root(3)/4 * b^2 = A

differentiate and solve for b

(9/8 + root(3)/2)b - 15/2 = 0

the numbers aren't gonna be pretty. just need to do it.

I think the only thing that's making it seem more complicated than it is is the way you expressed the area of an equilateral triangle.
Ethelis
Profile Blog Joined September 2008
United States2396 Posts
Last Edited: 2009-01-21 02:32:53
January 21 2009 02:32 GMT
#11
this

+ Show Spoiler +
[image loading]
Disabled gamer - Diamond 3 (LoL) D+ Rank scrublord on BW. Bisu doesnt need DTs, He uses probes. just ask Flash.
FragKrag
Profile Blog Joined September 2007
United States11552 Posts
January 21 2009 02:32 GMT
#12
ahhh I see. Thanks! I must have missed geometry when the teacher was talking about it :o

and Micronesia.. i didn't understand your post : (
*TL CJ Entusman #40* "like scissors does anything to paper except MAKE IT MORE NUMEROUS" -paper
rredtooth
Profile Blog Joined December 2008
5459 Posts
January 21 2009 02:33 GMT
#13
On January 21 2009 11:32 xMiragex wrote:
this

+ Show Spoiler +
http://img.4chan.org/b/src/1232496239730.jpg

Lol linking a 4chan image is the stupidest thing you can do since they go 404 so quickly.
[formerly sponsored by the artist formerly known as Gene]
Athos
Profile Blog Joined February 2008
United States2484 Posts
January 21 2009 02:35 GMT
#14
I fucking hate this shit; I think I just bombed a test on this :/
Nitrogen
Profile Blog Joined February 2008
United States5345 Posts
January 21 2009 02:36 GMT
#15
On January 21 2009 10:32 mahnini wrote:
1a2a3a HAHAHAHAHAHHAA


ROFL

On January 21 2009 11:33 redtooth wrote:
Show nested quote +
On January 21 2009 11:32 xMiragex wrote:
this

+ Show Spoiler +
http://img.4chan.org/b/src/1232496239730.jpg

Lol linking a 4chan image is the stupidest thing you can do since they go 404 so quickly.


i can think of a lot of things more stupid than that
UNFUCK YOURSELF
micronesia
Profile Blog Joined July 2006
United States24664 Posts
January 21 2009 02:45 GMT
#16
On January 21 2009 11:32 FragKrag wrote:
ahhh I see. Thanks! I must have missed geometry when the teacher was talking about it :o

and Micronesia.. i didn't understand your post : (

That's ok... I wasn't that clear since I did it quickly. I'm not sure if my method of using the perimeter is better or not anyway.
ModeratorThere are animal crackers for people and there are people crackers for animals.
FragKrag
Profile Blog Joined September 2007
United States11552 Posts
January 21 2009 03:33 GMT
#17
ok, problem 2:

Pink Pig is in the muddy field 4 mi from the nearest point, P, on Farm Road 5. She wants to get to Sally's Slop House on that road, 7 mi to the right of point P in the shortest possible time. Since her maximum possible speeds are 3 mph in mud and 6 mph on the road, she mentally calculates that she must maintain maximum possible speed and intersect the road ___ mi to the right of point P.

It's a mental problem so no scratch paper or calc. T_T
I think it incorporates related rates :x

updated OP
*TL CJ Entusman #40* "like scissors does anything to paper except MAKE IT MORE NUMEROUS" -paper
Hippopotamus
Profile Blog Joined October 2004
1914 Posts
January 21 2009 04:01 GMT
#18
Well, taking an intuitive approach to this problem is better than solving an ugly equation. Consider a unit equilateral triangle. That is, each side = 1. The area by well known formulas is going to be the square root of 3 divided by 4 or about .433. The square of perimeter 3 is going to have each side equal to 3/4. So the area is 9/16. 9/16 > .433. Now we know that every square and equilateral triangle is similar to these so the square is always going to be a more efficient use of wire by that ratio if you're looking for area. So you should use it all on the square.
ninjafetus
Profile Joined December 2008
United States231 Posts
Last Edited: 2009-01-21 04:27:08
January 21 2009 04:24 GMT
#19
Question 2: sqrt(3) miles. (see edit)

MS paint awesomeness:
[image loading]


EDIT: oh, fuck, I put the pig 3 miles from the road instead of 4... the same idea works, but change sqrt(9 + x^2) to sqrt (16 + x^2) and rework it. x should end up being, uhh... sqrt(16/3).
Chromyne
Profile Joined January 2008
Canada561 Posts
January 21 2009 04:35 GMT
#20
A 4 miles, what's this mile you speak of?

Anyway, good luck with Optimization, my worst section of Calc in high school.
Soli Deo gloria.
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