Tuesday
September 30, 2014

Homework Help: Trigo!

Posted by help! on Saturday, September 19, 2009 at 8:35am.

Ok, i've tried this sum a million times and i cant get an answer. Somebody please check my workings and tell me what i did wrong! Thanks!

Given 4≤y≤6, find the value of y for which 2cos((2y)/3) + √3 = 0

Here's my workings:

2.666≤(2y/3)≤4
2cos(2y/3)=-√3
cos(2y/3)= -(√3)/2
ref angle: 2.6179
2y/3= 0.5236, 5.759

and i can't seem to continue, because the answers don't tally with the range i calculated. ???
the correct answer is supposed to be y=5.5

please tell me what i did wrong!

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