State whether the following statements are true or false. Give a short proof or a counterexample in support of your Answer:
a) A real-valued function of three variables which is continuous everywhere is differentiable.
b) The function f(x,y)=ln((x+y)/(x))f(x, y)=\ln \left(\frac{x+y}{x}\right) is not homogeneous function.
c) The cylindrical coordinates of the point whose spherical coordinates is (8,(pi)/(6),(pi)/(2))\left(8, \frac{\pi}{6}, \frac{\pi}{2}\right) is (8,(pi)/(6),0)\left(8, \frac{\pi}{6}, 0\right)
d) The unique solution y(x)y(x) of an ordinary differential equation:
(dy)/(dx)={[0″,”,” for “x < 0],[1″,”,” for “x >= 0]:}\frac{d y}{d x}= \begin{cases}0, & \text { for } x<0 \\ 1, & \text { for } x \geq 0\end{cases}
is a second order differential equation of degree 3 .
f) Differential equation:
cos x(d^(2)y)/(dx^(2))+(dy)/(dx)+xy^(2)=0\cos x \frac{d^2 y}{d x^2}+\frac{d y}{d x}+x y^2=0
in ]0,pi[] 0, \pi[ is a linear, homogeneous equation.
g) The total differential equation corresponding to the family of surfaces x^(3)z+x^(2)y=cx^3 z+x^2 y=c, where cc is a parameter is 3x^(2)dz+x(ydx+xdy)=03 x^2 d z+x(y d x+x d y)=0.
h) Differential equation:
5x^(2)y^(2)z^(2)=2px^(2)y^(2)+5qx^(2)y^(3)+2pz^(2)+9x^(2)y^(2)5 x^2 y^2 z^2=2 p x^2 y^2+5 q x^2 y^3+2 p z^2+9 x^2 y^2
is a semi-linear partial differential equation of first order.
2x(dy)/(dx)+y(6y^(2)-x-1)=02 x \frac{d y}{d x}+y\left(6 y^2-x-1\right)=0
b) The rate of change of the price of a commodity is proportional to the difference between the demand DD and the supply S\mathrm{S}. If D=alpha-bPD=\alpha-b P and S=c sin beta tS=c \sin \beta t, where a,b,ca, b, c and beta\beta are constants, determine P(t)P(t). It is given that at t=0,P=P_(0)t=0, P=P_0.
a) Find the differential equations of the space curve in which the two families of surfaces:
u=x^(2)+y^(2)+3z=c_(1)u=x^2+y^2+3 z=c_1
and v=zx+x^(2)+y^(2)=c_(2)v=z x+x^2+y^2=c_2
intersect.
b) Transform the given equation to Clairaut’s form and hence find its general solution:
xy(y-px)=x-pyx y(y-p x)=x-p y
Also find its singular solution, if it exists.
a) Find the envelope and the characteristic curves of the family of curves:
State whether the following statements are true or false. Give a short proof or a counterexample in support of your answer:
a) A real-valued function of three variables which is continuous everywhere is differentiable.
Answer:
The statement "A real-valued function of three variables which is continuous everywhere is differentiable" is false.
Justification
Continuity and differentiability are related but distinct properties of functions. While differentiability implies continuity, the converse is not necessarily true. A function can be continuous at a point but not differentiable at that point. This concept holds for functions of any number of variables, including functions of three variables.
Counterexample
A classic counterexample in the context of functions of a single variable is the absolute value function, f(x)=|x|f(x) = |x|, which is continuous everywhere but not differentiable at x=0x = 0.
For a function of three variables, we can construct a similar example. Consider the function
f(x,y,z)=sqrt(x^(2)+y^(2)+z^(2))f(x, y, z) = \sqrt{x^2 + y^2 + z^2}
This function represents the distance from the origin in three-dimensional space and is continuous everywhere. However, it is not differentiable at the origin (0,0,0)(0, 0, 0).
To see why, consider the partial derivative with respect to xx at the origin. The partial derivative is defined as the limit of the difference quotient:
This limit does not exist because it approaches 11 as hh approaches 00 from the right and -1-1 as hh approaches 00 from the left. Therefore, the function f(x,y,z)=sqrt(x^(2)+y^(2)+z^(2))f(x, y, z) = \sqrt{x^2 + y^2 + z^2} is not differentiable at the origin, even though it is continuous everywhere.
Conclusion
Thus, the statement is false. A real-valued function of three variables that is continuous everywhere is not necessarily differentiable everywhere.
b) The function f(x,y)=ln((x+y)/(x))f(x, y)=\ln \left(\frac{x+y}{x}\right) is not homogeneous function.
Answer:
To determine the truth of the statement "The function f(x,y)=ln((x+y)/(x))f(x, y) = \ln\left(\frac{x+y}{x}\right) is not a homogeneous function", we need to verify if the function is homogeneous of any degree. A function f(x,y)f(x, y) is homogeneous of degree nn if it satisfies:
for all scalars lambda\lambda. If a function satisfies this condition for some degree nn, it is homogeneous; otherwise, it is not.
For the given function f(x,y)=ln((x+y)/(x))f(x, y) = \ln\left(\frac{x+y}{x}\right), let’s substitute lambda x\lambda x and lambda y\lambda y and simplify:
Simplifying this expression will help us compare it with lambda ^(n)f(x,y)\lambda^n f(x, y) to determine if the function is homogeneous and, if so, of which degree. Let’s perform this calculation.
Upon simplifying the function f(lambda x,lambda y)=ln((lambda x+lambda y)/(lambda x))f(\lambda x, \lambda y) = \ln\left(\frac{\lambda x + \lambda y}{\lambda x}\right), we find that it simplifies to:
since lambda^(0)=1\lambda^0 = 1 for any lambda\lambda. This means that the function f(x,y)=ln((x+y)/(x))f(x, y) = \ln\left(\frac{x+y}{x}\right) is indeed homogeneous, but of degree n=0n = 0.
Thus, the statement "The function f(x,y)=ln((x+y)/(x))f(x, y)=\ln \left(\frac{x+y}{x}\right) is not a homogeneous function" is false, as the function is homogeneous of degree 0.
c) The cylindrical coordinates of the point whose spherical coordinates is (8,(pi)/(6),(pi)/(2))\left(8, \frac{\pi}{6}, \frac{\pi}{2}\right) is (8,(pi)/(6),0)\left(8, \frac{\pi}{6}, 0\right)
Answer:
The statement "The cylindrical coordinates of the point whose spherical coordinates are (8,(pi)/(6),(pi)/(2))\left(8, \frac{\pi}{6}, \frac{\pi}{2}\right) is (8,(pi)/(6),0)\left(8, \frac{\pi}{6}, 0\right)" is false. To demonstrate this, let’s convert the given spherical coordinates to cylindrical coordinates and compare.
Spherical Coordinates to Cylindrical Coordinates Conversion
The spherical coordinates (r,theta,phi)(r, \theta, \phi) are related to cylindrical coordinates (rho,phi,z)(\rho, \phi, z) by the following relationships:
rho=r sin(phi)\rho = r \sin(\phi)
z=r cos(phi)z = r \cos(\phi)
phi\phi (the angle in the xy-plane from the x-axis) remains the same in both coordinate systems.
Given the spherical coordinates (8,(pi)/(6),(pi)/(2))\left(8, \frac{\pi}{6}, \frac{\pi}{2}\right), where r=8r = 8, theta=(pi)/(6)\theta = \frac{\pi}{6}, and phi=(pi)/(2)\phi = \frac{\pi}{2}, we can convert them to cylindrical coordinates:
The angle phi\phi in cylindrical coordinates is the same as theta\theta in spherical coordinates, so phi=(pi)/(6)\phi = \frac{\pi}{6}.
Conclusion
Therefore, the cylindrical coordinates of the point whose spherical coordinates are (8,(pi)/(6),(pi)/(2))\left(8, \frac{\pi}{6}, \frac{\pi}{2}\right) are (8,(pi)/(6),0)\left(8, \frac{\pi}{6}, 0\right) in terms of (rho,phi,z)(\rho, \phi, z), not (8,(pi)/(6),0)\left(8, \frac{\pi}{6}, 0\right) in terms of (r,theta,z)(r, \theta, z). The statement is false because it incorrectly identifies the third component of the cylindrical coordinates as 00 instead of correctly identifying the second component as (pi)/(6)\frac{\pi}{6} and the third component as 00.
d) The unique solution y(x)y(x) of an ordinary differential equation:
(dy)/(dx)={[0″,”,” for “x < 0],[1″,”,” for “x >= 0]:}\frac{d y}{d x}= \begin{cases}0, & \text { for } x<0 \\ 1, & \text { for } x \geq 0\end{cases}
exists AA x inR\forall x \in \mathbb{R}.
Answer:
To determine whether the unique solution y(x)y(x) exists for the given ordinary differential equation (ODE)
for all x inRx \in \mathbb{R}, we can use the Lipschitz condition.
The Lipschitz Condition
The Lipschitz condition is a criterion used to determine the existence and uniqueness of solutions to ODEs. A function f(x,y)f(x, y) satisfies the Lipschitz condition in yy on a domain if there exists a constant LL such that for all y_(1),y_(2)y_1, y_2 in the domain,
In our case, the function f(x,y)f(x, y) representing the right-hand side of the ODE is piecewise constant: it is 0 for x < 0x < 0 and 1 for x >= 0x \geq 0. This function does not depend on yy, so the difference |f(x,y_(1))-f(x,y_(2))||f(x, y_1) – f(x, y_2)| is always 0 regardless of the values of y_(1)y_1 and y_(2)y_2. Therefore, the function trivially satisfies the Lipschitz condition in yy for any xx, with any Lipschitz constant L >= 0L \geq 0.
Conclusion
Since the function f(x,y)f(x, y) satisfies the Lipschitz condition, the Picard-Lindelöf theorem (or the existence and uniqueness theorem) guarantees that a unique solution y(x)y(x) exists for each initial value problem defined by this ODE. This holds for all x inRx \in \mathbb{R}.
where y^((n))y^{(n)} is the nnth derivative of yy with respect to xx, and a_(i)(x)a_i(x) are functions of xx only. The function g(x)g(x) on the right-hand side is the non-homogeneous term.
Homogeneous Differential Equation: A linear differential equation is homogeneous if g(x)=0g(x) = 0 for all xx.
Analysis of the Given Differential Equation
The given differential equation is:
cos x(d^(2)y)/(dx^(2))+(dy)/(dx)+xy^(2)=0\cos x \frac{d^2 y}{d x^2} + \frac{d y}{d x} + x y^2 = 0
The term cos x(d^(2)y)/(dx^(2))\cos x \frac{d^2 y}{d x^2} is linear because it involves the second derivative of yy multiplied by a function of xx only.
The term (dy)/(dx)\frac{d y}{d x} is linear because it is the first derivative of yy without any multiplication by yy or its derivatives.
However, the term xy^(2)x y^2 is not linear because it involves yy raised to the second power, which is a non-linear term.
Conclusion
Since the equation includes the non-linear term xy^(2)x y^2, it is not a linear differential equation. Consequently, it cannot be a homogeneous linear differential equation either. Therefore, the statement is false. The differential equation is non-linear due to the presence of the y^(2)y^2 term.
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\(a^2=b^2+c^2-2bc\:Cos\left(A\right)\)
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