23a. cos 120degrees = -cos 60degrees = -1/2
cos=x/r=negative/1=negative
In homework for number 23a. you have to find the exact value of the expression. The first step to solving this problem is you have to find what quadrant 120degrees is in. It is in the second quadrant. Then you have to find out if your answer is going to be positive or negative. In order to do this you look at the quadrants and figure it out. In the first quadrant both x and y are positive. In the second quadrant the x is negative and the y is positive. In the third quadrant both the x and y are negative. In the fourth quadrant the x is positive and the y is negative. Now that you know all the 120 falls in the second quadrant and the x is negative so your answer will be negative. Next, you have to subtract 180 by 120 and you end up with 60. So your answer you have now is negative cos of 60 degrees. Then last, you remember from your brain the cos of 60 is 1/2. So the answer you get is negative cos of 60.
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