Solve all of them for brianiest

What is not true about kl and mn? what is mn? (show your work)

What can you determine about the solutions of this system

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?