How do you evaluate cos^(-1) [cos((-5pi)/3)]cos1[cos(5π3)]?

2 Answers
Jul 21, 2017

(-5pi)/35π3

Explanation:

cos^-1[cos( (-5pi)/3)]cos1[cos(5π3)]

Let cos^-1[cos( (-5pi)/3)] = thetacos1[cos(5π3)]=θ , then

cos theta = cos( (-5pi)/3)cosθ=cos(5π3)

:. theta = (-5pi)/3

:.cos^-1[cos( (-5pi)/3)] = (-5pi)/3 [Ans]

Jul 22, 2017

cos^-1(cos((-5pi)/3)) = pi/3

Explanation:

Here's the trick to this problem: cos^-1(theta) is only defined on the interval 0 le theta le pi.

So, what this problem is asking us to do is find the angle between 0 and pi whose cosine is the same as the cosine of (-5pi)/3.

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Adding 2pi to an angle doesn't change any of its trig functions, so we can say that:

cos((-5pi)/3) = cos((-5pi)/3 + 2pi)

cos((-5pi)/3) = cos(pi/3)

And since pi/3 is between 0 and pi, we can say that:

cos^-1(cos((-5pi)/3)) = pi/3

Final Answer