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Partial and Total Derivatives

Functions of two variables
If three variables x, y, z are so related that the value of z depends upon the values of x and y, then z is called the function of two variables x and y and this is denoted by z = f(x, y) 

Partial derivatives of first order 
Let z = f(x, y) be a function of two independent variables x and y. If y is kept constant and x alone is allowed to vary then z becomes a function of x only. The derivative of z with respect to x, treating y as constant, is called partial derivative of z with respect to x and is denoted by
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical EngineeringSimilarly the derivative of z with respect to y, treating x as constant, is called partial derivative of z with respect to y and is denoted by
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering
Thus Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering are called first order partial derivatives of z

Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

Partial derivatives of higher order

Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering
In general, Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

Solved Examples 1: First order partial derivative of u = yx is
Solution : 
u = yx
Treating y as constant
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering
treating x as constant
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

Euler’s theorem on Homogenous functions

If u is a homogenous function of degree n in x and y then
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical EngineeringSince u is a homogenous function of degree n in x and y it can be expressed as
u = xn f
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

Adding (1) and (2) we get
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

Note: Euler’s theorem can be extended to a homogenous function of any number of variables. Thus if u is a homogenous function of degree n in x, y and z then Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

Solved Examples 2: If Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering show that Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering.
Solution: 
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

f (x, y) is a homogenous function of degree -2 in x and y
By Euler’s theorem, we have
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

Composite functions

(i) If u = f (x, y) where x = ϕ(t), y = Ψ(t)
then u is called a composite function of (the single variable) t and we can find du/dt .

(ii) If z = f(x, y) where x = ϕ(u, v), y = Ψ(u, v)
then z is called a composite function of (2 variables) u & v so that we can find Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

Total derivative of composite functions

If u is a composite function of t, defined by the relations
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

Solved Example 3: Find du/dt when u = x when u = xy2 + x2 y, x = at2, y = 2at.
Solution:
The given equations define u as a composite function of t.
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering
= 2a3 t3 (5t + 8)
Also u = xy2 + x2 y = at2 . 4a2 t2 + a2 t4 . 2at = 4at4 + 2a3 t5 
Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering

The document Total Derivative, Gradient, Divergence, and Curl | Engineering Mathematics for Mechanical Engineering is a part of the Mechanical Engineering Course Engineering Mathematics for Mechanical Engineering.
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