Solve for the y-Intercept: Curve Through (4/9,1) with Slope of -2/y3

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Homework Help Overview

The problem involves finding the y-intercept of a curve that passes through the point (4/9, 1) and has a slope defined by the expression -2/y³. The discussion centers around understanding the nature of the curve and the implications of the given slope.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the initial misunderstanding of treating the slope as constant and the need to recognize that the slope is dependent on y. There is an exploration of separating variables to solve the differential equation dy/dx = -2/y³. Some participants attempt to derive the equation of the curve and question how to proceed after finding the constant of integration.

Discussion Status

The discussion is active, with participants exploring different interpretations of the problem. Some have derived a potential equation for the curve and are questioning how to use it to find the y-intercept. There is no explicit consensus on the final approach, but guidance has been offered regarding the use of the point provided in the problem.

Contextual Notes

Participants note the requirement to find a numerical solution for the y-intercept, while also grappling with the implications of the slope's complexity. There is mention of the need to solve for the curve before determining the y-intercept.

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Homework Statement


Find the y-intercept of the curve that passes through the point (4/9,1) and whose slope at (x,y) is −2/y3.


Homework Equations





The Attempt at a Solution


woudnt the y-int be y=-2x/y3 + 5.5 ?
i got it by plugging the slope into m in the equation y=mx+b
then plugged in the given points to get b..
but the question wants a numerical solution :S
 
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Slimsta said:
i got it by plugging the slope into m in the equation y=mx+b

That's the equation of a straight line, with constant slope, m. The slope you are given isn't constant, it depends on y. Your curve is not a straight line.

What is the slope of any curve f(x) at the point (x,f(x))?...Use that.
 
That makes no sense whatsoever. I think you know that. They want the y intercept of the curve. I think you have to solve for the curve. Whatever happened to the "separable equations" part of your post title? Shouldn't you separate dy/dx=(-2/y^3) and solve it??
 
Dick said:
That makes no sense whatsoever. I think you know that. They want the y intercept of the curve. I think you have to solve for the curve. Whatever happened to the "separable equations" part of your post title? Shouldn't you separate dy/dx=(-2/y^3) and solve it??

okay so i get dy/dx=(-2/y^3)
==> y^3 dy = -2dx
==> (y^4)/4 = -2x + C
==> y= sqrt4(-8x + 4C)

now what do i do?
 
Slimsta said:
okay so i get dy/dx=(-2/y^3)
==> y^3 dy = -2dx
==> (y^4)/4 = -2x + C
==> y= sqrt4(-8x + 4C)

now what do i do?

They gave you a point that it passes through, so find, the constant C.
 
rock.freak667 said:
They gave you a point that it passes through, so find, the constant C.

well if i plug (9/4, 1) in, i get C=4.75
from there y= sqrt4(-8x + 19)
now what do i do with this?
 
So you have a curve passing through the point given, with the slope that they require. What does the question tell you to do with it?
 
Office_Shredder said:
So you have a curve passing through the point given, with the slope that they require. What does the question tell you to do with it?

i got 2.087
once i got y = (-8x + 19)^0.25
y-int is when x=0 so y=19^0.25 = 2.087

its weird how the slope they gave us don't even help.. its there to make the question scary i guess.
 
You used the slope to figure out what y was as a function of x. How can you plug in x=0 if you don't know what the function is?
 

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