The circumference of a circle can be found using this formula:

\(\displaystyle C = 2 \pi r\)

Where \(\displaystyle \pi \approx 3.14\)

Substitute \(\displaystyle r = 6\) and use \(\displaystyle \pi = 3.14\), since we are rounding:

\(\displaystyle C = 2 \cdot 3.14 \cdot 6 = 37.68\)

Round this to \(\displaystyle 37.7\).

We will convert all linear distances to feet and all times to minutes.

The tires have diameter 30 inches, which is equal to \(\displaystyle 30 \div 12 = 2.5\) feet. Their circumference will be \(\displaystyle \pi\) times this, or \(\displaystyle 2.5 \pi\) feet.

The car moves at a rate of 25 miles per hour, which is equivalent to 

\(\displaystyle 24 \times \frac{5,280}{ 60} = 2,112\) feet per minute.

The number of revolutions each tire makes is the distance traveled divided by the circumference of the tire, which is

\(\displaystyle \frac{ 2,112 }{2.5 \pi} \approx \frac{ 2,112 }{7.8540} \approx 269\).

Each tire revolves about 269 times in the course of one minute.

The equation of circumference of a circle can be found using \(\displaystyle C=2\pi r\). By substituting in our radius of 7 we can solve for C.

\(\displaystyle C=2(7)\pi\)

\(\displaystyle C=14\pi\)

The equation for the circumference of a circle is \(\displaystyle C=2r\pi\), where \(\displaystyle r\) is the radius of the circle. The radius is half the diameter, or 5 inches.

\(\displaystyle C=2(5)\pi\)

\(\displaystyle C=10\pi\)

\(\displaystyle \pi=3.14\)

\(\displaystyle C=10(3.14)\)

\(\displaystyle C=31.4\)

The equation for circumference of a circle is \(\displaystyle C=2\pi r\).

Plug in the given radius value and solve:

\(\displaystyle C=2(3)\pi\)

\(\displaystyle C=2(3)(3.14)\)

\(\displaystyle C=18.84\)

To find the circumference given the diameter we use the equation: 

\(\displaystyle C=D\pi\)

Then we substitute 40 in for our diameter:

\(\displaystyle C=40\pi\)

The equation of the circumference of a circle is as follows:

\(\displaystyle C=2\pi r\)

Now we substitute in our circumference and solve for radius:

\(\displaystyle 24\pi=2\pi r\)

\(\displaystyle 12=r\)

We use the equation for the circumference of a circle: \(\displaystyle C=2\pi r\)

Now we substitute in our radius of 4:

\(\displaystyle C=2(4)\pi\)

\(\displaystyle C=8\pi\)

To solve this, you should first figure out your radius.  Remember that the area of a circle is defined as:

\(\displaystyle A=\pi r^2\)

For your data, you know that this is:

\(\displaystyle 81\pi=\pi r^2\)

Solving for \(\displaystyle r\), you get:

\(\displaystyle 81=r^2\), or

\(\displaystyle r=9\)

Now, recall that the circumference of a circle is defined as:

\(\displaystyle C=2\pi r\)

For your data, this is:

\(\displaystyle C=2\pi * 9 = 18\pi\)

A \(\displaystyle 90\) degree angle represents one fourth of a full circle.  Therefore, the total area of this circle is \(\displaystyle 4 * 4\pi\) or \(\displaystyle 16\pi\).  Now, recall your area formula:

\(\displaystyle A=\pi r^2\)

For your data, this means:

\(\displaystyle 16\pi = \pi r^2\)

Solving for \(\displaystyle r\), you get:

\(\displaystyle 16=r^2\) or \(\displaystyle r=4\)

Now, the circumference of a circle is defined as:

\(\displaystyle C=2\pi r\)

For your data, this is:

\(\displaystyle C=2*4*\pi=8\pi\)