# trigonometry

• Oct 25th 2009, 01:58 AM
alexandrabel90
trigonometry
hi!
Can someone teach me how to solce arccos(cos(7Pi/ 6))?

i tried to change 7pi/ 6 into the principal angle but what i thought of was:

cos (7pi/6)= -cos (pi/6)

although it is in the principal angle, i do not know how to continue after that.

thanks!
• Oct 25th 2009, 02:03 AM
Quote:

Originally Posted by alexandrabel90
hi!
Can someone teach me how to solce arccos(cos(7Pi/ 6))?

i tried to change 7pi/ 6 into the principal angle but what i thought of was:

cos (7pi/6)= -cos (pi/6)

although it is in the principal angle, i do not know how to continue after that.

thanks!

HI

it would be $\frac{5\pi}{6}$

cos (7pi/6)= -cos (pi/6)=cos (5pi/6)

Lets say

$\cos^{-1} \cos (\frac{5\pi}{6})=x$

$\cos (\frac{5\pi}{6})=\cos x$

So now , cancel the 'cos' , thus $x=\frac{5\pi}{6}$

Note that its different when its $\cos \cos^{-1}$ something
• Oct 25th 2009, 02:28 AM
alexandrabel90
but i thought that way that you did only applies to when the angle is in the principal range of 0 to 180 degrees?

so i was trying to convert 7pi/6 into the principal angle of cos x where x is in the principal range so that i can use the method that you have just used..
• Oct 25th 2009, 02:36 AM