trig/quadrant equation (1 Viewer)

[alez]

ok stuck here
2cos2x+1=0 between -pi and pi inclusive
answer is -2pi/3,-pi/3,pi/3,2pi/3
i really dont get the negative quadrant work
not sure how it works
in quadrant -1, are all positive or just cos?
is it going the reverse way around?
or do you just solve like normal but make it negative

 

[lolokay]

2cos2x+1=0
2cos2x= -1
cos2x = -1/2
2x = 2pi/3, 4pi/3 +/- (2pi)k
x = pi/3, 2pi/3 +/- (pi)k

for -pi <= x <= pi:
x = -2pi/3, -pi/3, pi/3, 2pi/3

quadrant -1 is quadrant 4 (just think about what the first quadrant would be if you were drawing your angle in reverse direction) so only cos is positive there, but you don't need to worry about non1-4 quadrants.

 

[Slidey]

Also remember cos(-x)=cos(x) or cos(-pi)=cos(pi).