Posted by Jillian on Friday, December 9, 2011 at 2:05pm.
Remember you can add or subtract 2pi = 360° without changing the values of the trig functions.
Then recall that for sin,tan,ctn,csc, f(-x) = -f(x)
for cos, sec, f(-x) = f(x)
cos 360° = cos(360°-360°) = cos 0°
sec(7pi/2) = sec(7pi/2 - 4pi) = sec(-pi/2) = sec(pi/2)
tan(-9pi/2) = tan(-9pi/2 + 4pi) = tan(-pi/2) = -tan(pi/2)
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