## trig equation: find value of x in cos(3x)= -0.419 for 0<x<

Trigonometric ratios and functions, the unit circle, inverse trig functions, identities, trig graphs, etc.
zack
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Joined: Wed Feb 08, 2012 9:02 pm
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### trig equation: find value of x in cos(3x)= -0.419 for 0<x<

find the value of x
cos(3x)= -0.419 for 0<x<270

I do arc cos -0.419 which equals 114.77

and then to find the second angle in the 3rd quadrant

why do I need to minus 360 to get the degree in the 3rd quadrant?

why not add 90 to 114.77 to make it in the 3rd quadrant?

little_dragon
Posts: 226
Joined: Mon Dec 08, 2008 5:18 pm
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### Re: trig equation: find value of x in cos(3x)= -0.419 for 0<

find the value of x
cos(3x)= -0.419 for 0<x<270

I do arc cos -0.419 which equals 114.77

and then to find the second angle in the 3rd quadrant

why do I need to minus 360 to get the degree in the 3rd quadrant?

why not add 90 to 114.77 to make it in the 3rd quadrant?
look @ the graph
the 1st one is 0+114.77 (114.77 in from the left)
the 2nd one is 360-114.77 (114.77 back from the right)
doing +90 doesnt work w/ the graph fo what cos does