ECON1010: MATHEMATICS FOR ECONOMICS 2017
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SUMMER TERM EXAMINATIONS 2017
ECON1010: MATHEMATICS FOR ECONOMICS
SECTION A
1. Let
「 1 ] 「 2 ] 「1]
| | | | | |
Verify that any two of the three 3-vectors u,v,w are orthogonal. Find scalars 入, p,vsuch that [入u pv vw] is an orthogonal matrix.
2. Suppose
z = x ln(1+ xy) .
Use the small increments formula to find an approximation to the change in z when x changes from 1 to 1+ 编x and y changes from 1 to 1+ 编y where 编x and 编y are small. Now suppose you are additionally given
x = 1 + t, y = t 2 .
Use the chain rule to find dz / dt when t = 1.
3. Show that the function
x2 - 2xy +5y2 - 10x + 2y
is convex.
Find its global minimum value.
4. Find the general solution of the differential equation
dy 4
Also find the solutions which satisfy y=1 when t=0 and y=0 when t=1 and sketch them on the same axes.
5. State Shephard’s lemma.
Suppose a firm’s cost function is given by
2(1)一a yb
where wi and y denote the price of the i-th input and the output level respectively and K, a andb are positive constants with 0 a 1 . Use Shephard’s lemma to find the conditional input demand functions.
6. Use l’Hôpital’s rule to find
x 入 一 y 入
lim 入0 入 .
7. Find the general solution ofthe difference equation yt +2 + 4yt +1 一 5yt = 2t .
8. Find the stationary solution of the system of differential equations
dx dy
dt dt
Find also the general solution and sketch the phase diagram and phase portrait. Is the stationary solution stable?
SECTION B
9. (a) Define the terms linear combination, linear dependence and linear independence as applied to vectors.
Suppose a,b and c are three vectors in Rn . Show that a,b and c are linearly dependent if
and only if there exist scalars α,β and γ, not all zero, such that
αa+βb+γc=0 .
State the generalisation of this result to the case of k vectors in Rn .
(b) Define the term echelon matrix and say what the 4 types of echelon matrix are. Hence show that the number of solutions of the simultaneous linear equations Ax = b is 0, 1 or infinity where A is an mn matrix, x is a vector in Rn and b is a vector in Rm .
(c) Use the results in (a) and (b) to show that if we have a set of more than n vectors in Rn , these vectors must be linearly dependent.
10. Explain what is meant by a homogeneous function of 2 variables of degree h. Show that the partial derivatives of such a function are homogeneous of degree h 一 1 .
Show that the utility function
U(x, y) = xy ,
where α and β are positive constants, is homogeneous and state the degree of homogeneity, h. Verify that the two marginal utilities are homogeneous of degree h 一 1 .
For this utility function, show that the slope of the indifference curves is constant along the line
y = cx
where c is a positive constant. Draw a diagram to illustrate this result.
Now show the same result is true when the utility function is a general homogeneous function of 2 variables.
11. A consumer has a utility function
U(x1 , x2 ) = 1 2
Find the demand functions, explaining carefully each step of your argument.
12. A firm has L units of labour at its disposal. Its outputs are three different commodities. Producing x, y and z units of these commodities requires x2 , y2 and z 2 units of labour respectively.
Consider the problem
maximise (ax + by + cz) subject to x2 + y2 + z2 L
where a, b, c, , and are positive constants.
(i) Write down the Lagrangian and the Kuhn-Tucker conditions.
(ii) Explain why the Kuhn-Tucker conditions are sufficient for a solution to this problem.
(iii) Find the solution.
(iv) When a = 4, b = c = 1, = 1, = and = , show that the problem has the solution x = , y = and z = . Use the Lagrange multiplier to find the approximate change in the maximum value when L increases from 100 to 101.
13. (a) Find the general solution of the differential equation (t 2 +1) + 3ty = 6t .
(b) A special case of the Solow-Swan growth model leads to the differential equation
+ 6K = sAKa
where K denotes the capital stock and 6, s anda are constants satisfying the following restrictions
6 > 0, s > 0, 0 <a<1.
Show that if x = K1 a- , then x satisfies the differential equation
dx
Find the general solution of this equation and hence find the general solution for the capital stock. What happens to the capital stock as t )w?
14. (a) By considering eigenvectors, find the general solution of the system of differential equations
dx dy
dt dt
and show that it can be written in the form
「x] 「 5cost ] 「 5sint ]
| | = a| |+ b| |
where aand b are constants.
(b) For the system of differential equations
dx dy
dt dt
find the stationary solution and sketch the phase diagram.
Verify algebraically that the stationary point is a saddle point, find the equation of the stable branch and sketch the phase portrait.
2022-08-20