**#72**

Evaluate the following: 600e^-x = 300

I get to the part where it's -x=ln(1/2) but the correct answer is x=ln(2/1).

Could someone explain to me why it changes from 1/2 to 2/1?

**#81**

What is Cos(495 degrees)?

I understand the Quadrant thing where you go around 450 degrees which leaves the angle at 45 degrees.

The answer is -Cos(45 degrees) = -(√2/2)

It says it's a reference angle. Is that something we can figure out or is that something we have to memorize somewhere?

**#87**

This could be taken from a lot of problems (like #86 even).

6 people are running for 4 positions. How many different combinations can there be?

This is an easy problem. 6x5x4x3 = 360.

The part that I don't get, which almost seems irrelevant, is that in the answer it shows (6x5x4x3x2)/(2) and the 2s just cancel out.

Why do they have those extra "2s" and is there kind of example where they actually are significant?

**Thanks!**