Yahoo Answers: Answers and Comments for Why is cos(2x) negative when π/4 < x< π/2 ? Isnt it supposed to be positive when it is in the first quarter? [Mathematics]
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From Anonymous
enUS
Mon, 11 Mar 2019 22:42:25 +0000
3
Yahoo Answers: Answers and Comments for Why is cos(2x) negative when π/4 < x< π/2 ? Isnt it supposed to be positive when it is in the first quarter? [Mathematics]
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https://answers.yahoo.com/question/index?qid=20190311224225AAkewTA
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From Como: :
x is in first quadrant but 2x is in second...
https://answers.yahoo.com/question/index?qid=20190311224225AAkewTA
https://answers.yahoo.com/question/index?qid=20190311224225AAkewTA
Tue, 12 Mar 2019 17:53:29 +0000
:
x is in first quadrant but 2x is in second quadrant.

From alex: π/4 < x< π/2
> π/2 < 2x< π
...
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https://answers.yahoo.com/question/index?qid=20190311224225AAkewTA
Mon, 11 Mar 2019 23:59:57 +0000
π/4 < x< π/2
> π/2 < 2x< π
> cos(2x) <0

From Johnathan: cos(x) is positive, but so is sin(x). cos(2x)...
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Mon, 11 Mar 2019 22:47:05 +0000
cos(x) is positive, but so is sin(x). cos(2x) = cos^2(x)  sin^2(x); if pi/4 < x < pi/2, cos(x) < sin(x). Squaring both trig values, sin^2(x) is larger, and subtracting that from cos^2(x) yields a result less than 0.