The cost of making $x$ units is $C(x) = 500 + 10x + 0.5x^{2}$ rupees. Using the marginal cost at $x = 20$, about how much does the 21st unit cost to make?

Explanation

1. 01

   $C'(x) = 10 + x$, so $C'(20) = 30$.

2. 02

   The marginal cost at 20 estimates the cost of the next unit: about ₹30. (Exactly, $C(21) - C(20) = 30.5$.)