Given that $cos(x) = -3/4$ ; what is the value of $x$ ?

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Simply the question itself, in my book the answer should be $pi/3$ but I can't remember how to solve it.
Or maybe the book is wrong in this case...

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Answer 1 · 2018-04-03T01:11:18

$cos(x)=-3/4$

$rArrx=arccos(-3/4)$

$rArrcolor(red)(x~~+-2.418858406)$ (in radians)

$-3/4$ is not one of the special values of the basic trigonometric functions, so it is difficult to express $x$ exactly in this case.

The so-called "special angles" and their corresponding outputs for the cosine function are as follows:

$x=0->cosx=1$

$x=pi/6->cosx=sqrt3/2$

$x=pi/4->cosx=sqrt2/2$

$x=pi/3->cosx=1/2$

$x=pi/2->cosx=0$