Which expressions are polynomials?

select each correct answer.

4x+x√−1

35x²

7x^7−x^−5

x/5+2

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?