MHB Which angle has a larger cosine value?

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The discussion centers on comparing the cosine values of two expressions: cos(B - π/2) and cos(θ + π/2). The initial calculations suggest that cos(θ + π/2) is greater, but the textbook claims the opposite. Clarification arises around the interpretation of the cosine values, with a suggestion that the problem may have been misread regarding the definitions of cos(B) and cos(θ). Participants also question the quadrant placements of the angles, which could affect the cosine values. The conversation highlights the importance of accurately interpreting mathematical expressions and definitions.
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We are given the following:

Let B = beta

cos B = cos (2_/6)/5

cos θ = cos 3/4

Which angle is larger:

cos (B - π/2) or cos (θ + π/2)?

I found cos (B - π/2) to be about 0.557.

I found cos (θ + π/2 to be about 0.731.

So, 0.731 > 0.557.

My answer is cos (θ + π/2) > cos (B - π/2).

Book's answer is cos (B - π/2) > cos (θ + π/2).

Why is my answer wrong?
 
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what is this value supposed to be?

(2_/6)/5
 
skeeter said:
what is this value supposed to be?

See picture.

View attachment 7938
 

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recheck the problem ... are you sure it's not

$\cos{\beta} = \dfrac{2\sqrt{6}}{5}$ and $\cos{\theta} = \dfrac{3}{4}$

instead of

$\cos{\beta} = \cos\left(\dfrac{2\sqrt{6}}{5}\right)$ and $\cos{\theta} = \cos\left(\dfrac{3}{4}\right)$

?
 
Yes, you are right. So, why is the textbook right? Why is my answer wrong?
 
RTCNTC said:
Yes, you are right. So, why is the textbook right? Why is my answer wrong?

one more note regarding the problem statement ...

Which angle is larger:

cos (B - π/2) or cos (θ + π/2)?

$\cos\left(\beta - \dfrac{\pi}{2}\right)$ and $\cos\left(\theta + \dfrac{\pi}{2}\right)$ are not angle values ... maybe which value of cosine is larger?

one more question, does the problem statement say anything about which quadrant(s) $\beta$ and $\theta$ terminate?
 
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skeeter said:
one more note regarding the problem statement ...
$\cos\left(\beta - \dfrac{\pi}{2}\right)$ and $\cos\left(\theta + \dfrac{\pi}{2}\right)$ are not angle values ... maybe which value of cosine is larger?

one more question, does the problem statement say anything about which quadrant(s) $\beta$ and $\theta$ terminate?

I need to get back to you. I will look in the textbook.
 
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