Quadrants and Angles (page 2 of 3)
Sections: Introduction, Worked Examples (and Sign Chart), More Examples
My picture doesn't need to be exact or "to scale". I just need the general idea of what quadrant I'm in and where the angle θ ("theta") is.
Then the Pythagorean Theorem gives me the length r of the hypotenuse:
And that's all I need for finding my ratios! To find my answers, I can just read the number from my picture:
For this exercise, I need to consider the x- and y-values in the various quadrants, in the context of the trig ratios. I don't need to find any actual values; I only need to work with the signs and what I know about the ratios and the quadrants.
The tangent ratio is y/x, so the tangent will be negative when x and y have opposite signs. This occurs in the second quadrant (where x is negative but y is positive) and in the fourth quadrant (where x is positive but y is negative). So the sign on the tangent tells me that the end of the angle is in QII or in QIV.
The sine ratio is y/r, and the hypotenuse r is always positive. So the sine will be negative when y is negative, which happens in the third
and fourth quadrants.
So the tangent is negative in QII and QIV, and the sine is negative in QIII and QIV. The overlap between the two solutions is QIV, so:
The terminal side of the angle θ lies somewhere in QIV.
The thought process for the exercise above leads to a rule for remembering the signs on the trig ratios in each of the quadrants. In the first quadrant, all the values (x, y, and r) are positive, so All the trig ratios are positive. In the second quadrant, the x-values are negative, so x/r and y/x are negative; only y/r is positive, so only the Sine is positive in QII. In the third quadrant, each of x and y is negative, so x/r and y/r are negative; only y/x is positive, so only the Tangent is positive in QIII. In the fourth quadrant, the y-values are negative, so y/r and y/x are negative; only x/r is positive, so only the Cosine is positive in QIV.
Some people remember the letters using the word "ACTS", but that's the reverse of normal (anti-clockwise) trigonometric order. Others remember the letters with the word "CAST", which is the normal rotational order but doesn't start in the usual (first-quadrant) starting place. To start in the usual spot and rotate in the usual direction, still others use the mnemonic "All Students Take Calculus" (which is a bit ironic when you're in a trig class). Use whichever method works best for you.
From the sign on the cosine value, I only know that the angle is in QII or QIII. That's why they had to give me that additional specification: so I'd know which of those quadrants I'm in.
The Pythagorean Theorem gives me:
Original URL: http://www.purplemath.com/modules/quadangs2.htm Copyright 2009 Elizabeth Stapel; All Rights Reserved.
Copyright 2009 Elizabeth Stapel; All Rights Reserved.