Q5*) Write down an immediate first integral for the Euler-Lagrange equation for the integral
I =
= F(x, y, y″) dx.
Hence write down a first integral of the Euler-Lagrange equation for the integral
I
1 = √(xy ² + x³y²)
dx.
Find the general solution of this ordinary differential equation, seeking first the complementary function and
then the particular integral. (Hint: the ODE is of homogeneous degree. And, for the particular integral, try
functions proportional to log x.)
You are provided with three 2D data points, p1, p2 and p3. Solving A C = B for C provides youwith the coefficients of a natural cubic spline curve that interpolates these points.Additionally, you have been given A and B, but some elements are missing. Moreover, the last two rowsof A are entirely absent. Your task is to determine and fill in the missing elements. For the last two rows,enforce a zero tangent at the beginning (in p1) and a not-a-knot boundary condition in p2. The matricesA and B are given as follows:Explain how to find the entries of A and B . How would you adapt these matrices if the data pointswere 3D? What if your spline should go through five data points? How many “extra rows” would there thenbe (with “extra” meaning “in addition to securing C2-continuity”)?
Q2*) In question P3 we showed that a minimal surface of revolution is given by revolution (about the x-axis)
of the catenary, with equation y = C cosh ((x – B)/C).
-
(a) Suppose, without loss of generality, that the catenary passes through the initial point
P = (x1,y1) = (0, 1). First deduce an expression for the one-parameter family of catenaries passing
through point P. Next calculate the value of x at which y takes its minimum value. By using the inequality
cosh > √2
(you might like to think about how to prove this), show that there are points Q for which it is impossible to find
a catenary passing through both P and Q. In particular, show that it is impossible to find a catenary joining
the points (0, 1) and (2, 1).
(b) A minimal surface of revolution can be realised experimentally by soap films attached to circular wire
frames (see this link and this link for examples). The physical reason for this is that the surface tension, which
is proportional to the area, is being minimised.…
Chapter 10 Solutions
Finite Mathematics for Business, Economics, Life Sciences and Social Sciences
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