Math geniuses, test your skill.

[Alienwho]

3 walls are added to a wall that is allready built. the side and back cost $10 per foot and the front wall $20. The area is 108 sq. ft. Minimize the cost.

Questions:

Write the formulas you use.

Tell how to minimize the cost.

How long is the $20 wall?

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One more question

Find dy/dt.

-2xy^3+3x^2=-1

I need the WORK on these, so I can study how it's done and do it correctly on the test.

 

[lawaris]

<<

--------

One more question

Find dt/dt.

-2xy^3+3x^2=-1

I need the WORK on these, so I can study how it's done and do it correctly on the test. >>




The question is either wrong or the ans. is 1

 

[b0mbrman]

let s = side
let b = back
let f = front

We know:
total cost = 10s + 10b + 20f
b = f
s * f = 108

Shuffle the third eq. around
s = 108/f

Substitute
total cost = 10(108/f) + 10f + 20f
= 1080*f^-1 + 30f

Derive
d total cost/df = -1080*f^-2 + 30

Set to 0
-1080*f^-2 + 30 = 0
-1080/f^2 = -30
1080 = 30f^2
36 = f^2
6 = f

 

[lawaris]

hey ........admit that the question was misprinted ........I feel like a fool

 

[Capn]

Is this a homework problem?

You're only hurting yourself in the end

You're gonna be at work some day, and your boss is gonna come and say: "Alienwho! minimize the cost of this room right now!" And where will you be then huh?

 

[b0mbrman]

I think this is wrong

-2xy^3 + 3x^2 = -1
-2xy^3 = -1 - 3x^2
y^3 = 1/2x + 3x/2
y = (h)^(1/3)
dy/dt = 0?

 

[Alienwho]

<< hey ........admit that the question was misprinted ........I feel like a fool >>


Oh sorry, it was dy/dt, not dt/dt, haha. My bad.

Thanks bomberman you rock!



<< Is this a homework problem? >>


Nope, my teacher doesn't believe in homework. I just have a test coming up and need to know how to do it for the test! (looks like my teachers philosophy of being anti homework works in the end, eh?)

 

[Alienwho]

b0mberman I *think* dy/dt is supposed to be done somehow like this.

-2xy^3+3x^2=-1
(-2x)(dy/dt)+(y^3)(-6x(dy/dt))+9x(dy/dt)=0
uhm, then I don't know.

 

[goodoptics]

<< b0mberman I *think* dy/dt is supposed to be done somehow like this.

-2xy^3+3x^2=-1
(-2x)(dy/dt)+(y^3)(-6x(dy/dt))+9x(dy/dt)=0
uhm, then I don't know. >>




or could it be like this,
dy/dt=dy/dx*dx/dt = (-2dx/dt)y^3+(-6xy^2)dy/dt+(6x)dx/dt=0

rearranged,
dy/dt=[(-3x+y^3)/(-3xy^2)]dx/dt

 

[HomeBrewerDude]

shouldn't your thread title be: "math geniuses, do my homework?"

 

[Pepsi90919]

HEHE