Function

**boy47** · November 8th 2008, 04:05 AM

Is: $g:[-1,1]=>[0,\pi]$ defined by: $g(x)=arccos(x)$ The value of $g=-\frac{1}{2}$ is:

a) $\frac{2\pi}{3}$
b) $-\frac{2\pi}{3}$
c) $\frac{\pi}{3}$
d) $-\frac{\pi}{3}$
e) $\frac{\pi}{6}$

**millerst** · November 9th 2008, 02:33 PM

You're question, from what I can gather, asks: What is the value of the cosine function in radians at -1/2 if the function has a range of [-1, 1] and a domain of 0 to Pi.

Take a look at the image below, Pi/2 occurs when y=0, and pi occurs when y= -1. Since we know that the point y=-1/2 lies between pi/2 and pi it cannot be pi/6 or pi/3. And radians are not negative since the domain is not between 0 and -pi. Therefore the answer has to be 2pi/3.

If you're in doubt try cos of 120° or cos of 2pi/3 in your calculator, you'll get the answer -0.5 or -1/2.

**Plato** · November 9th 2008, 03:27 PM

Originally Posted by boy47

Is: $g:[-1,1]=>[0,\pi]$ defined by: $g(x)=arccos(x)$ The value of $g=-\frac{1}{2}$ is:
a) $\frac{2\pi}{3}$
b) $-\frac{2\pi}{3}$
c) $\frac{\pi}{3}$
d) $-\frac{\pi}{3}$
e) $\frac{\pi}{6}$

Your question asks: $\cos (?) = - \frac{1}{2}$ .
The answer to that question is: $\frac{{2\pi }}{3}\,\& \, - \frac{{2\pi }}{3}$ .
BUT the domain of the arccosine function is $\left[ {0,\pi } \right]$ .
So what is the correct answer?

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