The Unit Circle has four quadrants, each separated by half of pi (Quadrant I: 0 to pi over 2, Quadrant II: pi over 2 to pi, Quadrant III: pi to 3pi over 2, Quadrant IV: 3pi over 2 to 2pi). The trig graphs' x-axis is separated also by these values. The trig graphs' x-axis is just the unit circle values in a linear perspective.
Whether a trig function is positive or negative in a certain quadrant (ASTC) is determined in the Unit Circle and this dictates how we draw the trig graphs. If the trig function value is positive, we draw it above the x-axis; below if it is negative.
Sin is positive in the first quadrant and the second quadrant; it is negative in the third and the fourth. If one goes counterclockwise, starting from quadrant 1, one will see that this pattern continues after the first rotation which is 2pi. Cosine has a similar pattern that repeats itself after 2pi. Tangent's ASTC pattern is: postive in the 1st, negative in the second, positive in the 3rd and negative in the fourth. This pattern, as you can see, repeats itself after the second quadrant which is represented by pi. Cotangent has the same pattern as tangent.
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