The cost function of a firm $C (x) = 4 x^{2} - x + 70$ . Find the marginal cost when $x = 3$ .

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Solution

The correct option is B 23
$\Rightarrow$ We have, $C (x) = 4 x^{2} - x + 70$ .

$\Rightarrow$ Marginal cost = $\frac{d}{d x} C (x)$

$\Rightarrow$ Marginal cost = $\frac{d}{d x} (4 x^{2} - x + 70)$

$\Rightarrow$ Marginal cost = $2 \times 4 x - 1 = 8 x - 1$
$\Rightarrow$ Substitute value of $x = 3$ ,
$\Rightarrow$ Marginal cost = $8 \times 3 - 1 = 23$
$∴$ $M a r g i n a l c o s t = 23$ .

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