Circle the equation with the rate of change 3

Np4=12np2 with clear explanation.

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?

Urgent? will give brainliest to the first correct answer what is the area of the figure?