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[Calculus] Cobb-Douglas Production Function

2021-10-06 19:11 作者:AoiSTZ23  | 我要投稿

By: Tao Steven Zheng (郑涛)

【Problem】

The Cobb-Douglas production function is an econometric model formulated by American economist Paul Douglas (1892 - 1976) and mathematician Charles Cobb (1875 - 1949) in 1927. They modelled total production as a function of labour L and capital investment K:


P(L%2CK)%20%3D%20c%20%7BL%7D%5E%20%7B%5Calpha%7D%20%7BK%7D%5E%7B%5Cbeta%7D%20

where c%20%3E%200%2C%20%5Calpha%20%3E%200%2C%20%5Cbeta%20%3E%200.

If the budget constraint is mL%20%2B%20nK%20%3D%20p, where m%2Cn%20 are the unit cost of labour and capital. Determine the labour and capital values that maximizes production in terms of %20%5Calpha%2C%20%5Cbeta%2C%20m%2C%20n%2C%20p.

【Solution】

This problem requires the use of Lagrange multipliers. Consider the following functions:

P(L%2CK)%20%3D%20c%20%7BL%7D%5E%20%7B%5Calpha%7D%20%7BK%7D%5E%7B%5Cbeta%7D

g(L%2CK)%20%3D%20mL%20%2B%20nK%20-%20p%20

Take the following partial derivatives in order to generate the equations for %5Cnabla%20P%20%3D%20%5Clambda%20%5Cnabla%20g.

%5Cfrac%7B%5Cpartial%20P%7D%7B%5Cpartial%20L%7D%20%3D%20c%20%5Calpha%20%7BL%7D%5E%7B%5Calpha%20-%201%7D%20%7BK%7D%5E%7B%5Cbeta%7D

%20%5Cfrac%7B%5Cpartial%20P%7D%7B%5Cpartial%20K%7D%20%3D%20c%20%5Cbeta%20%7BL%7D%5E%7B%5Calpha%7D%20%7BK%7D%5E%7B%5Cbeta%20-%201%7D%20

%5Cfrac%7B%5Cpartial%20g%7D%7B%5Cpartial%20L%7D%20%3D%20m

%5Cfrac%7B%5Cpartial%20g%7D%7B%5Cpartial%20K%7D%20%3D%20n

Therefore, we get the equations

%20c%20%5Calpha%20%7BL%7D%5E%7B%5Calpha%20-%201%7D%20%7BK%7D%5E%7B%5Cbeta%7D%20%3D%20%5Clambda%20m

c%20%5Cbeta%20%7BL%7D%5E%7B%5Calpha%7D%20%7BK%7D%5E%7B%5Cbeta%20-%201%7D%20%3D%20%5Clambda%20n%20

Isolate for %5Clambda for both of the above equations and isolate for either K or L (here I chose K).


%5Clambda%20%3D%20%5Cfrac%7Bc%20%5Calpha%7D%7Bm%7D%20%7BL%7D%5E%7B%5Calpha%20-%201%7D%20%7BK%7D%5E%7B%5Cbeta%7D%20%3D%20%5Cfrac%7Bc%20%5Cbeta%7D%7Bn%7D%20%20%7BL%7D%5E%7B%5Calpha%7D%20%7BK%7D%5E%7B%5Cbeta%20-%201%7D

Subsequently,

K%20%3D%20%5Cfrac%7B%5Cbeta%20m%7D%7B%5Calpha%20n%7D%20L

Substitute this value of K into the budget constraint to solve for L. Then use the newfound value of L to solve for K.

mL%20%2B%20n%5Cleft(%5Cfrac%7B%5Cbeta%20m%7D%7B%5Calpha%20n%7D%20L%5Cright)%20%3D%20p

%20mL%20%5Cleft(1%20%2B%20%5Cfrac%7B%5Cbeta%7D%7B%5Calpha%7D%20%5Cright)%20%3D%20p

L%20%3D%20%5Cfrac%7B%5Calpha%7D%7B%5Calpha%20%2B%20%5Cbeta%7D%20%5Ccdot%20%5Cfrac%7Bp%7D%7Bm%7D%20

K%20%3D%20%5Cfrac%7B%5Cbeta%20m%7D%7B%5Calpha%20n%7D%20%5Ccdot%20%5Cfrac%7B%5Calpha%7D%7B%5Calpha%20%2B%20%5Cbeta%7D%20%5Ccdot%20%5Cfrac%7Bp%7D%7Bm%7D%20


Therefore, the solution set that maximizes production is

%5Cleft(L%2C%20K%20%5Cright)%20%3D%20%5Cleft(%5Cfrac%7B%5Calpha%7D%7B%5Calpha%20%2B%20%5Cbeta%7D%20%5Ccdot%20%5Cfrac%7Bp%7D%7Bm%7D%2C%20%5Cfrac%7B%5Cbeta%7D%7B%5Calpha%20%2B%20%5Cbeta%7D%20%5Ccdot%20%5Cfrac%7Bp%7D%7Bn%7D%20%5Cright)

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