the monthly revenue for printer ink for a large office supply store is r(x) =-x^2 +12x+13, where x is the price of an ink cartridge in dollars and r(x) is in hundreds of dollars. what is the maximum monthly ink revenue

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?