Chapter 4 Quadratic Functions and Factoring - 4.7 Complete the Square - 4.7 Exercises - Problem Solving - Page 290: 64

decreasing the price by $\$ 10$

$y=(70-x)(50+x)=-x^2+20x+3500=-(x^2-20x)+3500=-((x-10)^2-100)+3500=-(x-10)^2+100+3500=-(x-10)^2+3600$ Thus the vertex of the graph is at $(10,3600)$, and the shop can maximize its revenue by decreasing the price by $ \$10$.