Once again, why was I able to divide by this expression? This expression by the Pythagorean identity, which really comes out of the unit circle definition of trig functions, this is equal to 1, so I have not changed the value of this expression. What makes this interesting is if I want to introduce a sine divided by a cosine, I could just divide the numerator and the denominator by cosine squared. Let's do that. Let's divide the numerator by cosine squared of y and divide the denominator by cosine squared of y, or multiply each of them by 1 over cosine squared of y. What's that going to give us? The numerator, these characters are going to cancel.