the trick to that is the term "All Students Think Clearly". ASTC stands for "all", "sine", "tangent", and "cosine". the term defines which terms are positive in which quadrants, and it goes in order from quadrant 1-4. so, in quadrant one, all terms (sin, cos, and tan including their inverses) are positive. in quadrant 2, sin and its inverse (cosecant) is positive. in quadrant 3, tangent and its inverse (cotangent) is positive. and in quadrant 4, cosine and its inverse are positive.