# Find theta given known cos and sin...

## Homework Statement ## Homework Equations

cos 2theta = costheta^2 - sintheta^2

## The Attempt at a Solution

cos2theta = 1
2theta = 0, 2phi

but i get wrong answer.. how is it?

Ray Vickson
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## Homework Statement

View attachment 205960

## Homework Equations

cos 2theta = costheta^2 - sintheta^2

## The Attempt at a Solution

cos2theta = 1
2theta = 0, 2phi

but i get wrong answer.. how is it?

Well, given your values for ##\sin \theta## and ##\cos \theta## you should NOT get ##\cos 2 \theta = 1##. Check your algebra.

Well, given your values for ##\sin \theta## and ##\cos \theta## you should NOT get ##\cos 2 \theta = 1##. Check your algebra.
oh.. cos 2 theta = root 2 / 2
2theta = 45degrees
the sin is negative, theta must on quadrant 2 or 3
225 or 315 degrees.

phinds
Science Advisor
Gold Member
oops. never mind

mjc123
Science Advisor
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π
You get the "wrong" answer because the problem posits an impossible condition. Just use arccos and arcsin and you'll see they are talking about two different angles so it can't be right to call them both the same. For that matter, you don't even have to do any math; the signs alone tell you the angles are different.
No, with cos +ve and sin -ve, the angle must be in the 4th quadrant. All silly tom cats. (A calculator will give you different angles.)
θ = 2π - π/8 = 15π/8, and 2θ = 15π/4 (note the question asked for the answer as a multiple of π, not in degrees).

phinds
Science Advisor
Gold Member
No, with cos +ve and sin -ve, the angle must be in the 4th quadrant. All silly tom cats. (A calculator will give you different angles.)
θ = 2π - π/8 = 15π/8, and 2θ = 15π/4 (note the question asked for the answer as a multiple of π, not in degrees).
Yes, I had already deleted my post before you posted this

Ray Vickson
Science Advisor
Homework Helper
Dearly Missed
π
No, with cos +ve and sin -ve, the angle must be in the 4th quadrant. All silly tom cats. (A calculator will give you different angles.)
θ = 2π - π/8 = 15π/8, and 2θ = 15π/4 (note the question asked for the answer as a multiple of π, not in degrees).

Or, the angle could be ##-\pi/4##, assuming negative angles are allowed.

mjc123
Science Advisor
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The question said 0 ≤ θ < 2π.

But the answer also 15/4 pi.. Which is 337.5 degree

Ray Vickson
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But the answer also 15/4 pi.. Which is 337.5 degree

No, it isn't. ##2 \pi \leftrightarrow 360^o##, and ##15/4> 3> 2##, so ##15/4 \pi ## is a lot bigger than ##360^0##.

no
No, it isn't. ##2 \pi \leftrightarrow 360^o##, and ##15/4> 3> 2##, so ##15/4 \pi ## is a lot bigger than ##360^0##.
the key answer it is. the 2 theta allowed to be bigger than 2 pi. just theta < 2pi

SammyS
Staff Emeritus
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no

the key answer it is. the 2 theta allowed to be bigger than 2 pi. just theta < 2pi

Yes, but you said:
But the answer also 15/4 pi.. Which is 337.5 degree
As Ray said, that's not true. After all, (15/4)π ≠ 337.5° .

Perhaps you meant that
one of the answers is 2θ = (15/4)π , which means that θ = 337.5° ,​
so, of course, θ = (15/8)π < 2π .

But the restrictions say theta is defined on [0,2pie)?
Hence, 2theta=-pie/4.

From our restriction, this -pie/4 should be equal to 7pie/4 (sin is negative and cos is positive/ IV Quadrant).

Therefore 2theta=7pie/4
theta=7pie/8

Mark44
Mentor
But the restrictions say theta is defined on [0,2pie)?
Hence, 2theta=-pie/4.

From our restriction, this -pie/4 should be equal to 7pie/4 (sin is negative and cos is positive/ IV Quadrant).

Therefore 2theta=7pie/4
theta=7pie/8
You do know that there is a difference between "pie" and "pi" with the latter being the name of a Greek letter, right?

You do know that there is a difference between "pie" and "pi" with the latter being the name of a Greek letter, right?

typing with a new tablet... with auto correct...

You do know that making a post within a topic, that is not relevant to the discussion is considered trolling? That is a violation of PF rules of conduct.

mjc123
Science Advisor
Homework Helper
But the restrictions say theta is defined on [0,2pie)?
Hence, 2theta=-pie/4.

From our restriction, this -pie/4 should be equal to 7pie/4 (sin is negative and cos is positive/ IV Quadrant).

Therefore 2theta=7pie/4
theta=7pie/8
The restriction applies to θ, not 2θ. It is also θ that must be in the fourth quadrant, therefore θ = 7π/8 is not acceptable

ehild
Homework Helper
The restriction applies to θ, not 2θ. It is also θ that must be in the fourth quadrant, therefore θ = 7π/8 is not acceptable
The restriction applies to theta, but cos(2θ) is positive and sin(2θ) is negative, therefore 2θ is in the fourth quadrant or 2pi more. And the problem asks 2θ.

mjc123
Science Advisor
Homework Helper
2θ is indeed in the fourth quadrant, but so is θ, as cos θ is positive and sin θ negative, so 2θ must be in the "8th quadrant", as it were.

@mjc123 why θ = 2π - π/8 = 15π/8, and 2θ = 15π/4 ?
Why you substract π/8 from 2π?

π
No, with cos +ve and sin -ve, the angle must be in the 4th quadrant. All silly tom cats. (A calculator will give you different angles.)
θ = 2π - π/8 = 15π/8, and 2θ = 15π/4 (note the question asked for the answer as a multiple of π, not in degrees).
@mjc123 2 θ = is 45 degrees, theta = 22.5 degrees
So quadrant 4 is 360-22.5 = 337.5
So 2theta = 337.5 x 2 = 675 degrees (15/4)pi
Why can't i use directly 45degrees, so theta in quadrant 4 is 360-45 = 315 degrees
Since 315 and 675 have same cos value, why the answer is 675?

Never mind i get it. Thanks for all help

mjc123
Science Advisor
Homework Helper
@mjc123 why θ = 2π - π/8 = 15π/8, and 2θ = 15π/4 ?
Why you substract π/8 from 2π?
From the magnitudes of cosθ and sinθ, the basic angle is 22.5° (π/8). (If you have to do this without a calculator, from the magnitude of cos 2θ, 2θ = 45°.) However, since cosθ is positive and sinθ is negative, θ must be in the 4th quadrant, i.e. between 3π/2 and 2π.
Why can't i use directly 45degrees, so theta in quadrant 4 is 360-45 = 315 degrees
Since 315 and 675 have same cos value, why the answer is 675?
I assume you mean "2 theta in quadrant 4 is 360-45". You can't use this because (for the millionth time) θ must be in the 4th quadrant. So 2θ must be between 3π and 4π.

• Helly123
From the magnitudes of cosθ and sinθ, the basic angle is 22.5° (π/8). (If you have to do this without a calculator, from the magnitude of cos 2θ, 2θ = 45°.) However, since cosθ is positive and sinθ is negative, θ must be in the 4th quadrant, i.e. between 3π/2 and 2π.

I assume you mean "2 theta in quadrant 4 is 360-45". You can't use this because (for the millionth time) θ must be in the 4th quadrant. So 2θ must be between 3π and 4π.
Haha yes. Thanks

Mark44
Mentor
You do know that making a post within a topic, that is not relevant to the discussion is considered trolling?
At PF we endeavor to maintain a high-quality site for the pursuit of science and mathematics, one aspect of which is to not conflate the number ##\pi## (or pi) with the dessert whose name is pronounced the same. I am aware that you have a strong knowledge of mathematics, but because you wrote "pie" six times in your post, I believed that this deserved comment and was relevant to the discussion.
MidgetDwarf said:
That is a violation of PF rules of conduct.
As a mentor, I am very familiar with the forum rules. Hijacking a thread is a violation, but comments that relate directly to posts in the thread are neither off-topic nor hijacks.

• SammyS
The restriction applies to θ, not 2θ. It is also θ that must be in the fourth quadrant, therefore θ = 7π/8 is not acceptable

Thank you. I now fully understand. So silly to make that common mistake.