In calculus, instead of working in degrees which you are
probably already familiar with, we work in a different, yet equivalent,
angle
measurement. The measurements don't change in amount, but they are
expressed
a little bit differently. For example, the angle opposite to the
hypotenuse of a right triangle measures 90 degrees. We need to convert
this
to **radians**. To convert any angle in degrees to radians multiply the
angle in
degrees by

/180. So 90 * /180 = /2. So 90 degrees is
/2 radians.

You might be wondering why we work in radians instead of degrees. We are more familiar with degrees, but radians is a more natural way to work with angles. One degree is based on an arbitrary measurement, but radian measure is based on the circumference of a unit circle. A unit circle is a circle with radius equal to 1. The circumference of that circle is 2. So if we travel once around the whole circle, we have travelled 2 radians which is equal to 360 degrees. So when we are talking about a quarter of a circle which is 90 degrees we are multiplying 2 by 1/4 which is /2. Ta Da we have our radian masure. Now you know why we use radians in Calculus.

We will need to know some common ratios of sides of a right
triangle.

An easy way to remember them is with acronyms.

Also there are a number of trigonometric identities.
Some of them don't show up too much, but if you are going to remember
one, remember this one: