How do you find the exact value of cos [(-7pi)/6]?

1 Answer
Daniel L.
Apr 21, 2015

cos((-7pi)/(6))=-(sqrt(3)/2)

First step cos(-7/6pi)=cos(7/6pi) because cosine is an even function (for evry x f(-x)=f(x))

Now you have to reduce cos(7/6 pi) to a value of a functon of an angle smaller than pi/2 rad. To do this you use the formula
cos(pi+x)= - cos(x). So you get cos(7/6 pi)=-cos(pi/6)=-(sqrt(3))/2

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