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Solve the following example: If the total cost function is given by; C = 5x3 + 2x2 + 7; find the average cost and the marginal cost when x = 4.

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Question

Solve the following example: If the total cost function is given by; C = 5x3 + 2x2 + 7; find the average cost and the marginal cost when x = 4.

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Solution

Total cost function, C = 5x3 + 2x2 + 7

Average cost = `"C"/x`

 =`(5x^3 + 2x^2 + 7)/x`

= 5x2 + 2x + `7/x`

When x = 4,

Average cost = 5(4)2 + 2(4) + `7/4`

= 80 + 8 + `7/4`

= `(320 + 32+ 7)/4`

= `359/4`

Marginal cost = `("dC")/("d"x)`

=`"d"/("d"x)(5x^3 + 2x^2 + 7)`

= `5"d"/("d"x) (x^3) + 2"d"/("d"x)(x^2) + "d"/("d"x)(7)`

= 5(3x2) + 2(2x) + 0
= 15x2 + 4x
When x = 4, Marginal cost = `(("dC")/"dx")_ (x = 4)`
= 15(4)2 + 4(4)
= 240 + 16
= 256
∴ the average cost and marginal cost at x = 4 are `359/4` and 256 respectively.

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Rules of Differentiation (Without Proof)
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Chapter 9: Differentiation - Exercise 9.2 [Page 122]

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