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# If y = f (u) is a differential function of u and u = g(x) is a differential function of x, then prove that y = f [g(x)] is a differential function of x and dy/dx=dy/(du) xx (du)/dx - Mathematics and Statistics

#### Question

If y = f (u) is a differential function of u and u = g(x) is a differential function of x, then prove that y = f [g(x)] is a differential function of x and dy/dx=dy/(du) xx (du)/dx

#### Solution

Let δx be a small increment in the value of x.
Since u is a function of x, there should be a corresponding increment u in the value of u.
Also y is a function of u.
there should be a corresponding increment δy in the value of y.
Consider,

(deltay)/(deltax)=(deltay)/(deltau)xx(deltau)/(deltax)

Taking lim_(deltax->0) on both sides, we get

lim_(deltax->0)(deltay)/(deltax)=lim_(deltax->0)(deltay)/(deltau)xxlim_(deltax->0)(deltau)/(deltax) [because deltax->0,deltau->0]

lim_(deltax->0)(deltay)/(deltax)=lim_(deltau->0)(deltay)/(deltau)xxlim_(deltax->0)(deltau)/(deltax) ....(i)

But lim_(deltax->0)(deltau)/(deltax)=(du)/(dx) "exists and is finite."

also  lim_(deltau->0)(deltay)/(deltau)=(dy)/(du)

limits on R.H.S. of (i) exist and are finite.
Hence, limits on L.H.S. should also exist and be finite

lim_(deltax->0)(deltay)/(deltax)=dy/dx " exisit and finite"

(dy)/(dx)=(dy)/(du)xx(du)/(dx)

Is there an error in this question or solution?