### Video Transcript

The circumference of a circle as a function of its radius is given by πΆ of π equals
two ππ. Express the radius of a circle as a function of its circumference denoting π of πΆ and
then find π if the circumference is 36π.

Hereβs what we know. That finding the circumference in terms of π equals two ππ. This formula helps us find the circumference if weβre given the
radius. We wanna transform this function into something that looks like this: π of πΆ. And that
would help us find the radius if we were given the circumference. So where should we start? We
wanna start by isolating this variable β getting the π by itself.

Right now, π is being multiplied by two π. I can use the multiplicative inverse of two π one over two π to multiply these
values together equals one. In other words, they cancel each other out: two π times one over two
π equals one. But the multiplication property of equality tells me that if I multiply by
something on one side of the equation, I have to multiply by the same thing on the other side.
So now we have to multiply the circumference by one over two π.

Now, we have the circumference divided by two π equals π. Because the circumference is no longer a function of the radius
anymore, weβre going to just write πΆ to represent the circumference. And because the radius is
now a function of the circumference, we can write π of πΆ. Just for clarity and because thatβs
how we usually write a function, Iβll move that π of πΆ to the left and say the radius as a
function of the circumference equals the circumference divided by two π; thatβs here.

Our next step will be to solve for the radius if weβre given a circumference of 36π. So weβve plugged in 36π
, where our circumference πΆ variable was 36 divided by two equals 18 and π divided by π
equals one. The radius of a circle with a circumference 36π equals 18. And here is our
reordered formula that helped us find that.