Why Does Rewriting an Equation Change the Implicit Derivative Result?

In summary, when you differentiate an equation with respect to x, you can lose the "2"s in the numerator if you rewrite the equation. Rewriting the equation changed the answer in this case.
  • #1
thharrimw
115
0
when i took the derivative of X^2*Y^2-2x=6 i didn't get the same thing that i got when i took the derivative of y^2=(6+2x)/x^2 and all i did was rewrite the origional equation. why did rewriting the equation change my answer?
 
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  • #2
thharrimw said:
when i took the derivative of X^2*Y^2-2x=6 i didn't get the same thing that i got when i took the derivative of y^2=(6+2x)/x^2 and all i did was rewrite the origional equation. why did rewriting the equation change my answer?

Could you show us your work? That would make it easier for us to help.
 
  • #3
not rewritting i gou
(x^2)(Y^2)-2x=3
(x^2)(2y)(y')+2x(y^2)-2=0
(x^2)(2y)(y')=2-(2x)(Y^2)
y'=(1-x(y^2))/((x^2)2y))
 
  • #4
when i rewrote it i got y^2=(3+2x)/(x^2)
and my derivitive was (x-3)/(y(x^3))
 
  • #5
I'm not tracking your math. Are you differentiating the equations with respect to x? It would help if you used LaTex to be explicit about your differentiations...

[tex]\frac{dy}{dx} = etc.[/tex]


http://en.wikipedia.org/wiki/Implicit_function

.
 
  • #6
thharrimw said:
not rewritting i gou
(x^2)(Y^2)-2x=3
(x^2)(2y)(y')+2x(y^2)-2=0
(x^2)(2y)(y')=2-(2x)(Y^2)
y'=(1-x(y^2))/((x^2)2y))
Okay, [itex]x^2y^2- 2x= 3 has derivative [itex]2xy^2+ 2x^2y y'- 2= 0 so [itex]2x^2y y'= 2- 2xy^2[/itex] [itex]y'= (2-2xy^2)/(2x^2y)[/itex], the "2"s cancel and y'= (1- xy^2)/(x^2y). You seem to have lost the "2" in the numerator.

thharrimw said:
when i rewrote it i got y^2=(3+2x)/(x^2) and my derivitive was (x-3)/(y(x^3))
No, solving for y2 gives [itex]x^2 y^2=6+ 2x[/itex] so [itex]y^2= (6+ 2x)/x^2[/itex] The derivative of [itex]y^2[/itex], with respect to x, is 2y y' and the derivative of [itex](6+2x)/x^2[/itex] using the quotient rule is [itex][(2)x^2- (6+2x)(2x)]/(x^4)= [-2x^2- 12x]/(x^4)= -(2x^2+ 12x)/(x^4)= -2(x+ 6)/(x^3)[/itex]. [itex]2yy'= -2(x+6)/x^3[/itex] gives [itex]y'= -(x+1)/yx^3[/itex].

Think those aren't the same? Replace the y2 in the numerator of the first derivative above with (2x+6)/x2 and simplify.
 
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  • #7
[itex]2yy'= -2(x+6)/x^3[/itex] gives [itex]y'= -(x+1)/yx^3[/itex].

wouldn't that simplify to [itex]y'= -(x+6)/yx^3[/itex].
 
  • #8
thharrimw said:
when i rewrote it i got y^2=(3+2x)/(x^2)
and my derivitive was (x-3)/(y(x^3))

When I rewrite this I get (-x-3)/(y*x^3). Could that be the problem
 

Related to Why Does Rewriting an Equation Change the Implicit Derivative Result?

What is a "weird implicit derivative"?

A "weird implicit derivative" refers to the derivative of a function that cannot be expressed explicitly in terms of its variables. This typically occurs when the function is defined implicitly, meaning that one variable is defined in terms of another.

How is the "weird implicit derivative" different from a regular derivative?

The "weird implicit derivative" differs from a regular derivative in that it cannot be found by simply applying the power rule or chain rule. Instead, implicit differentiation must be used to find the derivative of the function.

Why is the "weird implicit derivative" important?

The "weird implicit derivative" is important because it allows us to find the instantaneous rate of change of a function even when it cannot be expressed explicitly. This is useful in many applications, such as in physics and engineering.

What are some common techniques for finding the "weird implicit derivative"?

Some common techniques for finding the "weird implicit derivative" include using the chain rule, product rule, and quotient rule. Another approach is to solve the implicit equation for one variable and then take the derivative with respect to the other variable.

Can the "weird implicit derivative" be negative?

Yes, the "weird implicit derivative" can be negative. This indicates that the function is decreasing in that particular region. However, it is important to note that the sign of the "weird implicit derivative" does not necessarily correspond to the sign of the regular derivative.

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