Okay, so I'm working with L'Hopitals rule and indeterminate forms.. and I ran across this problem in my homework. I get the right answer, only that it is positive instead of negative and I can't figure out why it's negative.

The problem:

limit (sec(3x)cos(5x)) x-> (pi/2) from the left

since that is in infinity * 0 form... I did:

= cos(5x) / (1/sec(3x)) = 0/0

Great... I can now use L'Hopitals...

= -5sin(5x) / -3sin(3x) = 5/3

Negatives cancel right?? So now where did I go wrong? The answer should be negative according to my textbook, Wolfram Alpha, and MS Math.