Recall that the price elasticity of demand E is the percentage rate of decrease of demand divided by the percentage increase of price, given by the formula. E = - dq . P dp q We are already given the formula q = -10p + 4,420 for the demand of smartphones (in millions). First, we find the derivative dq dp dq -10 dp Next, substitute the values for and q into the formula for the price elasticity demand. dp E = - dq . P dp q P = X -10p + 4,420 10 p -10p + 4,420 10p Therefore, the price elasticity demand for smartphones is E = -10p + 4420