# Need tips

• Sep 30th 2012, 09:07 AM
Petrus
Need tips
hello this maybe is wrong to ask but i wounder if any1 got a tips for a page that can help u if u got problem with geometry :/ maybe webblesion..?
well im reading a course named analysis and got problem with this geometry basicly i havent understand this (yet) well i can give u exemple what we doing.. Calculate https://webwork.math.su.se/webwork2_...3ffde44dc1.png. The answer may not contain trigonometric functions.
basicly we have been using calculater before but without calculate im really bad on this because i havent understand it (yet) so tips plz?
• Sep 30th 2012, 09:22 AM
skeeter
Re: Need tips
$\arccos\left(-\frac{1}{2}\right)$ is the angle in the interval $[0,\pi]$ whose cosine is $-\frac{1}{2}$ ...

$\arccos\left(-\frac{1}{2}\right) = \frac{2\pi}{3}$

Pauls Online Notes : Algebra/Trig Review - Inverse Trig Functions
• Sep 30th 2012, 09:24 AM
MarkFL
Re: Need tips
Think of this as:

$\theta=\arccos\left(-\frac{1}{2} \right)$ where $0\le\theta\le\pi$

We can rewrite the equation as:

$\cos(\theta)=-\frac{1}{2}$

What special angle in the given range makes this true?
• Sep 30th 2012, 09:29 AM
Petrus
Re: Need tips
Quote:

Originally Posted by skeeter
$\arccos\left(-\frac{1}{2}\right)$ is the angle in the interval $[0,\pi]$ whose cosine is $-\frac{1}{2}$ ...

$\arccos\left(-\frac{1}{2}\right) = \frac{2\pi}{3}$

Pauls Online Notes : Algebra/Trig Review - Inverse Trig Functions

hello skeeter!
well -1/2 does it mean u go backway ehmm maybe bad explain but ima try explain it on a easy way because idk other way to explain, not best english...
uk how the cloak goes, that way so it should be 0,2pi? can u also explain a litle more how u got it to 2pi/3
• Sep 30th 2012, 10:03 AM
Petrus
Re: Need tips
Quote:

Originally Posted by MarkFL2
Think of this as:

$\theta=\arccos\left(-\frac{1}{2} \right)$ where $0\le\theta\le\pi$

We can rewrite the equation as:

$\cos(\theta)=-\frac{1}{2}$

What special angle in the given range makes this true?

cos x = cos (x + 2kx)
• Sep 30th 2012, 10:10 AM
MarkFL
Re: Need tips
That's true for any x, where k is an integer.

You are expected to know that:

$\cos\left(\frac{2\pi}{3} \right)=-\frac{1}{2}$
• Sep 30th 2012, 10:18 AM
Petrus
Re: Need tips
ohh ok but if i would solve this problem with that formel i wouldent know how to do it.. the thing is i got problem to decide that k...