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January 6th, 2017, 08:45 AM  #11 
Senior Member Joined: Dec 2015 From: Earth Posts: 134 Thanks: 20 
$\displaystyle dy=\sqrt{f'(C_1)dy + \int \limits_{C_1}^{z}\int \limits_{C}^{y}d^{2}y d{z}}$ 
January 6th, 2017, 11:26 AM  #12 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 975 Thanks: 78 
Given f'(x) By definition, u and v are differentials if they are related by: u=f'(x)v for given x. For example, dy=f'(x)dx. If f'(x) =10, and dx=.001, dy=.01. dx=1000, dy =10000 There are properties of the differential stemming from considerations of calculus, but the above is the basic definition. EDIT If z=f(x,y), u,v,w are differentials if they are related by: w = $\displaystyle \frac{\partial z}{\partial x}u + \frac{\partial z}{\partial y}v$ Last edited by zylo; January 6th, 2017 at 11:41 AM. 
January 6th, 2017, 12:42 PM  #13 
Senior Member Joined: Aug 2012 Posts: 1,164 Thanks: 256  
January 6th, 2017, 03:44 PM  #14  
Senior Member Joined: Mar 2015 From: New Jersey Posts: 975 Thanks: 78  Quote:
$\displaystyle dz = f_{x}dx + f_{y}dy$ $\displaystyle dx = (1/f_{x})dz  (f_{y}/f_{x})dy = \frac{\partial x}{\partial z}dz + \frac{\partial x}{\partial y}dy$ $\displaystyle \frac{\partial x}{\partial z} = 1/f_{x}$ $\displaystyle \frac{\partial x}{\partial y} =  (f_{y}/f_{x})$ Unfortunately, the trend seems to be to replace useful basic concepts with unintelligble jargon which becomes the basis of unintelligible discussion for the "in" crowd.  
January 7th, 2017, 09:58 AM  #15 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 975 Thanks: 78 
w = w(x,y,z) = 0 is equation of a surface. Small excursions dx,dy,dz from a point on the surface are related approximately by $\displaystyle dw = w_{x}dx + w_{y}dy + w_{z}dz = 0$, which is equation of the tangent plane at the point. As points on the plane, the differentials dx,dy,dz can have any value. If you solve w = 0 for y = y(x,z), $\displaystyle dy = (w_{x}/w_{y})dx(w_{z}/w_{y})dz = \frac{\partial y}{\partial x}dx + \frac{\partial y}{\partial z}dx$ $\displaystyle \frac{\partial y}{\partial x} = w_{x}/w_{y}$ $\displaystyle \frac{\partial y}{\partial z} = w_{z}/w_{y}$ 
January 8th, 2017, 09:55 PM  #16 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 975 Thanks: 78 
SUMMARY: $\displaystyle \Delta y = y'\Delta x + \epsilon$ dy = y'dx, any dx. Definition of derivative y' at x: y' = Lim $\displaystyle \frac{\Delta y}{\Delta x}$ Having defined y', increments $\displaystyle \Delta y= f(x+\delta x)f(x)$, and $\displaystyle \Delta x$ are related by: $\displaystyle \Delta y = y'\Delta x + \epsilon$, Where Lim $\displaystyle \epsilon / \Delta x$ = 0. Definition of differentials dy and dx at x: dx = $\displaystyle \Delta x$ dy = y'dx Last edited by zylo; January 8th, 2017 at 10:21 PM. 
January 9th, 2017, 11:00 AM  #17 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 975 Thanks: 78 
$\displaystyle dy = \frac{dy}{dx}$dx is ambiguous. It is either an identity, b = $\displaystyle \frac{b}{a}$a, or a formula relating variables (differentials), db = b'da, where da is the independent variable (differential) and db is the dependent variable (differential), by definition, da arbitrary (.0001 or 10000 for ex) Partial Derivatives and Differentials. All d's in the following are differentials. Let u=u(x,y). Then $\displaystyle du = u_{x}dx + u_{y}dy$, The dependent variable (differential) du is a function of the variables (differentials) dx and dy, regardless of their source of variation: either independently assigned or functions of other variables. In the latter case, let x=x(v,w) and y=y(v,w). Then u = u(v,w) and $\displaystyle dx = x_{v}dv + x_{w}dw$ $\displaystyle dy = y_{v}dv + y_{w}dw$, and $\displaystyle du(v,w) = u_{x}(x_{v}dv + x_{w}dw) + u_{y} (y_{v}dv + y_{w}dw)$ where now v and w are indepentent and $\displaystyle du(v,w) = u_{v}dv + u_{w}dw$, so that: $\displaystyle u_{v}dv + u_{w}dw = u_{x}(x_{v}dv + x_{w}dw) + u_{y} (y_{v}dv + y_{w}dw)$ v,w,dv,dw independent, u dependent. Now let dw=0 and divide by dv $\displaystyle \neq$ 0, and let dv=0 and divide by dw $\displaystyle \neq$ 0, respectively. $\displaystyle u_{v} = u_{x}x_{v} + u_{y}y_{v}$ $\displaystyle u_{w} = u_{x}x_{w} + u_{y}y_{w}$ Shortcut (chain rule) Under above condtions: $\displaystyle du = u_{x}dx + u_{y}dy$ divide by dv and dw respectively: $\displaystyle u_{v} = u_{x}x_{v} + u_{y}y_{v}$ $\displaystyle u_{w} = u_{x}x_{w} + u_{y}y_{w}$ 

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definition, differential, rigorous 
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