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INFORMATIC PRINCIPLES Univariate Differential Calculus: Solutions to Review Questions £11.06
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INFORMATIC PRINCIPLES Univariate Differential Calculus: Solutions to Review Questions

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INFORMATIC PRINCIPLES Univariate Differential Calculus: Solutions to Review Questions

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Univariate Differential Calculus: Solutions to Review Questions
1-
Find the first derivative of the following functions


a) f ( x)  8 3 x 4  2 x 2  1

df ( x)
 4(12 x 3  4 x)(3x 4  2 x 2  1) 1/ 2
dx

x2 1
f ( x) 
b) 3x 3  2 x

df ( x) (2 x)(3x 3  2 x)  ( x 2  1)(9 x 2  2)

dx (3x 3  2 x) 2

4 x)
f ( x)  4 xe ( x
2
c)

df ( x)
 (4e ( x 4 x ) )  (4 x)(2 x  4)e ( x 4 x )
2 2


dx
2-
The total cost function for a manufacturer has the following form,

TC= 2Q3 - 45Q2 + 300Q where Q is the output level.

a)Find the level of output, which minimises the total cost,
b)Find the average cost at the production level, which minimises the cost.

a) FOC: MC  6Q 2  90Q  300  0  Q=10 , Q=5
SOC: MC '  12Q  90

Q=10 MC’=30>0 Minimum, therefore Q=10 minimises the Total cost
Q=5 MC’=-30<0 Maximum

b) Q=10  TC = 500  AC=50

3-
The total revenue and marginal cost functions of a manufacturing firm have been defined as
TR = 5900Q-10Q2 MC = 6Q2 -8Q +140
where Q is the output level. If the firms fixed cost is 845, Find:

a)The level of output, which maximises the total revenue,
b)The maximum revenue
c)The level of output, which maximises the firm’s profit,
d)The total cost when profit is maximised,
e)The average cost when profit is maximised.


a) FOC: MR = 5900 – 20Q = 0  Q=295
SOC: MR’ = -20 <0 therefore Q=295 is the level which maximises TR

b) Q=295  TR= 870,250




120

, c) We know that   TR  TC  5900Q  10Q 2  TC
FOC:
d 
dQ

  
 0  TR  TC   0  MR  MC  0  5900Q  10Q 2  6Q 2  8Q  140  0  

 6Q 2  12Q  5760  0

Q=-32 not acceptable Q=30 acceptable

d 2 d 2
SOC:  12Q  12  Q=30   372  0
dQ 2 dQ 2

 Q=30 maximises profit

d) We need to find the total cost when profit is maximised, but we know only MC and not TC i.e. we
know that

TC   MC  TC   6Q 2  8Q  140
We need to find the function that if we differentiate it will give us TC.

 
However basic differentiation rules state that: Q   1 , Q 2  2Q and, Q   3Q
3 2



Hence:    
TC  6Q 2  8Q  140  3Q 2  3Q 2  4(2Q)  140(1)

And the above can be written as: TC   Q3   Q3   4Q 2   140Q

Or: TC   2Q3  4Q 2  140Q
Which is TC  2Q3  4Q 2  140Q
 
   
TC   2 Q 3  4 Q 2  140Q 



 
(Check: TC   2Q  4Q  140Q  6Q  8Q  140 )
3 2 2



The final total cost function includes also a fixed cost of 845, So:
TC  2Q3  4Q 2  140Q  845
And, for Q =30:
TC(30) = 55,445

e) AC = TC/Q = 55,445/30=1,848.167

4-
Given the following demand function,

Q=100-24P

a)Find the price elasticity of demand at P=1 and P=3,
b)Find the price at which the price elasticity demand is unity,
c)What is the relationship between the price level in part (b) and the supplier’s revenue?

a)

P=1  Q=76 , P=3  Q=28


121

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