How do you find cos if tan=3/4?
In general the sign of sine, cosine or tangent is underdetermined when we know just one of the others. That's why we need the
For reference we should enumerate the forms like this that come up. They're pretty easy to write down when we think about the numerator and denominator as opposite, adjacent or hypotenuse as appropriate.
I consider those two equations equivalent; i.e.
That's all to say when I write the inverse trig functions below, they're just abbreviations for the question being asked here, e.g if