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**Macleef** *A music store predicts that every dollar increase in the price of any CD will cause sales to decrease by 10 000 units a year. The store now sells 300 000 CD's a year at $15 each.*

**a) ****Develop a model that represents the store's sales revenue**

$\displaystyle R(x) = x - 10 000$

$\displaystyle C(x) = 15x$

$\displaystyle AP(x) = \frac {(x - 10 000) - 15x}{x}$

$\displaystyle AP(x) = \frac {-14x - 10 000}{x}$

$\displaystyle AP(x) = -14 - \frac {10 000}{x}$

**b)** **Using this model, determine when the revenue will increase and decrease**

the revenue decreases everywhere

**c)** __What is the rate of change in revenue if price increases by $2?__

$\displaystyle C(x) = 17x$

$\displaystyle m = \frac {17x - 15x}{17 - 15}$

$\displaystyle m = \frac {5 100 000 - 4 500 000}{17 - 15}$

$\displaystyle m = 300$