Answers:
- sin = -5/13
- tan = 5/12
- csc = -13/5
- sec = -13/12
- cot = 12/5
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Explanation:
The angle theta is between pi and 3pi/2, excluding both endpoints.
This places theta in the third quadrant (Q3) between 180 degrees and 270 degrees. The third quadrant is in the southwest.
Plot point A at the origin. 12 units to the left of this point, will be point B. So B is at (-12,0). Then five units lower is point C at (-12,-5). Refer to the diagram below. Notice how triangle ABC is a right triangle.
The angle theta will be the angle BAC, or simply angle A.
Since cos(theta) = -12/13, this indicates that
AB = -12 = adjacent
AC = 13 = hypotenuse
Technically, AB is should be positive, but I'm making it negative so that we can then say
cos(angle) = adjacent/hypotenuse
cos(theta) = AB/AC
cos(theta) = -12/13
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If you apply the pythagorean theorem, you should find that BC = 5, which I'll make negative since we're below the x axis. Then we can say
sin(theta) = opposite/hypotenuse
sin(theta) = BC/AC
sin(theta) = -5/13
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If you divide sine over cosine, then you'll get 5/12. The 13's cancel out. This is the value of tangent.
Or you could say
tan(theta) = opposite/adjacent
tan(theta) = BC/AB
tan(theta) = (-5)/(-12)
tan(theta) = 5/12
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To find csc, aka cosecant, you apply the reciprocal to sine
sin = -5/13 which means csc = -13/5
sec, or secant, is the reciprocal of cosine
cos = -12/13 leads to sec = -13/12
and finally cotangent (cot) is the reciprocal of tangent
tan = 5/12 leads to cot = 12/5
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Note: everything but tan and cot is negative in Q3.
