# Value of this trigonometric expression

songoku
Homework Statement:
Given that sin A + sin B = 1 and cos A + cos B = 0, find the value of 12 cos 2A + 4 cos 2B
Relevant Equations:
Trigonometry
Let:
equation 1 : sin A + sin B = 1
equation 2 : cos A + cos B = 0

Squaring both sides of equation 1 and 2 then add the result gives me: cos (A - B) = -1/2

Then how to proceed? Thanks

Gold Member
We know from equation 1
$$0<A,B<\pi$$
so equation 2 says
$$A,B=\frac{\pi}{2}-\theta, \frac{\pi}{2}+\theta$$ where
$$0<\theta<\frac{\pi}{2}$$

• songoku
songoku
We know from equation 1
$$0<A,B<\pi$$
so equation 2 says
$$A,B=\frac{\pi}{2}-\theta, \frac{\pi}{2}+\theta$$ where
$$0<\theta<\frac{\pi}{2}$$
I get it. From there, we can get the value of A and B.

Thank you very much anuttarasammyak

Mentor
We know from equation 1
$$0<A,B<\pi$$
Edited...
No, this isn't true. Consider ##A = \frac \pi 6## and ##B = \frac{13\pi} 6##. Then ##\sin A + \sin B = 1##. This is just one of an infinite number of counterexamples.
anuttarasammyak said:
so equation 2 says
$$A,B=\frac{\pi}{2}-\theta, \frac{\pi}{2}+\theta$$ where
$$0<\theta<\frac{\pi}{2}$$
I've tried several different approaches, but none has been successful so far.

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songoku
Edit: B should read ##\frac{13\pi} 6##.
No, this isn't true. Consider ##A = \frac \pi 6## and ##B = \frac{7\pi} 6##. Then ##\sin A + \sin B = 1##.
I am not sure I understand your hint, but:
sin (π/6) + sin (7π/6) = sin (π/6) + sin (π + π/6) = sin (π/6) - sin (π/6) = 0 ?

Thanks

Last edited by a moderator:
Homework Helper
Gold Member
Hint: First focus to prove that ##\cos 2A=\cos 2B##. To prove this you can proceed relatively easily from ##\cos A=-\cos B## by squaring both sides.

Once you prove that ##\cos 2A=\cos 2B## then you can solve this relatively simple trigonometric equation and relate directly the angles A,B. Then use equation 1.

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• songoku
Mentor
I am not sure I understand your hint, but:
sin (π/6) + sin (7π/6) = sin (π/6) + sin (π + π/6) = sin (π/6) - sin (π/6) = 0 ?

Thanks
I have edited my earlier post.
What I meant was ##A = \frac \pi 6## and ##B = \frac {13\pi} 6##. Then ##\sin A + \sin B = 1##, but it's not true that both A and B are in the interval ##[0, \frac \pi 2]## as was earlier claimed.

Homework Helper
Homework Statement:: Given that sin A + sin B = 1 and cos A + cos B = 0, find the value of 12 cos 2A + 4 cos 2B
Relevant Equations:: Trigonometry

Let:
equation 1 : sin A + sin B = 1
equation 2 : cos A + cos B = 0
eq. 2 says that cosB=-cosA, From eq.1, it follows that both angles are in the interval (0,pi). Therefore B=pi-A. Substitute for B into eq. 1
sinA+sin(pi-A)=2sinA=1--->sinA=0.5 , A=30°, B=°150°.

• songoku and Delta2
Homework Helper
Gold Member
eq. 2 says that cosB=-cosA, From eq.1, it follows that both angles are in the interval (0,pi). Therefore B=pi-A. Substitute for B into eq. 1
sinA+sin(pi-A)=2sinA=1--->sinA=0.5 , A=30°, B=°150°.
I don't understand how you conclude that the angles are in the interval ##(0,\pi)##. My approach at the very end of day concludes the same result, that is ##\sin A=\sin B=0.5## but without making any conclusions on what are the exact values of the angles.

• songoku
songoku
I have edited my earlier post.
What I meant was ##A = \frac \pi 6## and ##B = \frac {13\pi} 6##. Then ##\sin A + \sin B = 1##, but it's not true that both A and B are in the interval ##[0, \frac \pi 2]## as was earlier claimed.
Sorry I still not fully understand. Isn't 13π/6 located in first quadrant? And I think post #2 claimed that A and B are in interval (0, π), not [0, π/2]

I don't understand how you conclude that the angles are in the interval ##(0,\pi)##. My approach at the very end of day concludes the same result, that is ##\sin A=\sin B=0.5## but without making any conclusions on what are the exact values of the angles.

This is what I concluded from hint in post#2:

From sin A + sin B = 1, both sin A and sin B must be positive because if either one is negative, the value of the other trigonometry function will more than 1 (which is impossible) so this means 0 < A , B < π

• Delta2
Homework Helper
My approach at the very end of day concludes the same result, that is ##\sin A=\sin B=0.5## but without making any conclusions on what are the exact values of the angles.
Yes, you noted that cos(A) = - cos(B) which directly means B= pi-+A. And the sine of both angles must be positive, so B=pi-A.
You overcomplicated the solution a bit by squaring and working with double angles.

• songoku and Delta2
Homework Helper
Gold Member
2022 Award
A=30°, B=150°.
Or vice versa. But because we are only interested in cos(2A) and cos(2B), it makes no difference.
I have the feeling there is some quick way that avoids finding the angles, but I don't see it.

• songoku
Homework Helper
Gold Member
Or vice versa. But because we are only interested in cos(2A) and cos(2B), it makes no difference.
I have the feeling there is some quick way that avoids finding the angles, but I don't see it.
The way i did it avoids finding the angles (i found only sines/cosines of angles/double angles) but its not quicker, its longer actually...

• songoku and Ibix
2022 Award
I have the feeling there is some quick way that avoids finding the angles, but I don't see it.
I managed it by writing ##\cos A=-\cos B## and ##\sin A=1-\sin B## and squaring both sides of each. This gives you ##\sin B## in a couple of lines.

• songoku, etotheipi and Delta2
Quick way: Note that ##|\cos A|=| \cos B |##. Therefore ##|\sin A|=\sqrt{1-\cos^2 A}=\sqrt{1-\cos^2B}=|\sin B|##. Since the sum of the sines is equal to +1, both of them must be positive and ##\sin A=\sin B=\frac{1}{2}##. Now $$\cos(2A)=1-2\sin^2A=1-2\left(\frac{1}{2}\right)^2=\frac{1}{2}=1-2\sin^2B=\cos(2B).$$
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