Fundamental question: we know that y = x dy/dx, or rather... y/x = dy/dx...

dinomight

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Jun 17, 2017
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I'm confused:
we know that y = x dy/dx, or rather... y/x = dy/dx,
suppose y = x^2,
dy/dx = 2x
y/x = 2x
y = (2x)x
y = 2x^2 ? contradiction?
 
I'm confused:
we know that y = x dy/dx, or rather... y/x = dy/dx,
suppose y = x^2,
dy/dx = 2x
y/x = 2x
y = (2x)x
y = 2x^2 ? contradiction?

Yes it is contradiction - and it says that:

y =x^2

is NOT a solution for:

y = x dy/dx ..... !
 
Are you saying that you think this equality is true for all x and y?
I think that the OP is saying just that. (To OP) Why do you think that equation is always true (called an identity)? YOU even found a counter example! You are thinking that y=dy and x=dx? If if y=x^2, then to compute the derivative, dy/dx, you just divide both sides by x? So there is no need to ever use the product rule, the quotient rule nor the chain rule since you can just divide by x? Please think about what I am saying!
 
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