Since the problem said "0deg < or = Incl < 180deg", -81.4 couldn't possibly be the correct answer, could it?
Hint: the tangent function is periodic with period 180 degrees.
I have a problem in the book asking for inclination with a given slope. I'm told to use formula m = tan(inclination) as long as (0deg < or = Incl < 180deg)
I use tan^-1(-6.691) and get -81.4 but the book says it's 98.5
Please help. Thanks.