# limit for angle

• June 19th 2013, 08:42 AM
Trefoil2727
limit for angle
State the limits for B if given that B is an obtuse angle and
a) tan2B is negative
b) tan4B is positive
c) tan2B is negative or tan 4B is positive
• June 19th 2013, 09:19 AM
Plato
Re: limit for angle
Quote:

Originally Posted by Trefoil2727
State the limits for B if given that B is an obtuse angle and
a) tan2B is negative
b) tan4B is positive
c) tan2B is negative or tan 4B is positive

@Trefoil2727, Why do you keep posting a list of question, but show no effort?

From the given we know $\frac{\pi}{2}. So any answer must be a subset of that.

a) if $\tan(2B)<0$ then it must be that $\frac{3\pi}{2}<2B<2\pi$

Now you do some work and show us.
• June 20th 2013, 08:28 PM
Trefoil2727
Re: limit for angle
Quote:

Originally Posted by Plato
@Trefoil2727, Why do you keep posting a list of question, but show no effort?

From the given we know $\frac{\pi}{2}. So any answer must be a subset of that.

a) if $\tan(2B)<0$ then it must be that $\frac{3\pi}{2}<2B<2\pi$

Now you do some work and show us.

b) tan4B >0, 270<4B<450 , 540<4B<630
67.5<B<112.5 , 135<B<157.5 ?

I really can't understand, sorry..
• June 21st 2013, 03:24 AM
Trefoil2727
Re: limit for angle
hah is that means that 360<4B<720?
so tan4B>0, 360<4B<(90+360), (180+360)<4B<(270+360)
90<B<112.5 , 135<4B<157.5 ?

tan2B is negative or tan 4B is positive means what?
• June 21st 2013, 05:24 AM
Plato
Re: limit for angle
Quote:

Originally Posted by Trefoil2727
b) tan4B >0, 270<4B<450 , 540<4B<630
67.5<B<112.5 , 135<B<157.5 ?
I really can't understand, sorry..

It is clear to me that you don't understand much of this, do you?

From the given it must be true that $90^o. Thus any answer you find must be in that range.

If $\tan(2B)<0$ then $270^o<2B<360^o$ or $135^o, which is in that range.

If $\tan(4B)>0$ then it could be $180^o<4B<270^o$ or $45^o, which is NOT in that range.

So we must take another course.

Suppose that $4B\in\text{quad } I$ or $360^o<4B<450^o$ then $\tan(4B)>0$.
Solving we get $90^o, which is in the range.