Question

Find the value of ${{\cos }^{-1}}\left( \cos 1540{}^\circ \right)$.

Answer 1

Hint: Simplify $\cos 1540{}^\circ $ so that you can use the property \[{{\cos}^{-1}}\left( \cos x \right)=x\]. For this, use the property that a trigonometric operation of the form $\cos \left( 360{}^\circ \times n+x \right)$ can be written as \[\cos \left( x \right)\]. Next use the property \[{{\cos }^{-1}}\left( \cos x \right)=x\] on the simplified expression to arrive at the final answer.

Complete step by step solution:
In this question, we need to find the value of ${{\cos }^{-1}}\left( \cos 1540{}^\circ \right)$.
We first need to identify that the range of the function \[{{\cos }^{-1}}\left( x \right)\] is between \[0{}^\circ \] and \[180{}^\circ \].
In our question, we are given \[1540{}^\circ \] which is not in this range. So, we cannot
directly write \[{{\cos }^{-1}}\left( \cos 1540{}^\circ \right)=1540{}^\circ \]. So, we need to
simplify \[1540{}^\circ \].
To find this value, we will first evaluate $\cos 1540{}^\circ $ and then we will come to the inverse part.
First, let us simplify $\cos 1540{}^\circ $
We can write 1540 as:
$1540=360\times 4+100$
So, we can write $\cos 1540{}^\circ $ as the following:
$\cos 1540{}^\circ =\cos \left( 360{}^\circ \times 4+100{}^\circ \right)$
Now, we know the property that a trigonometric operation of the form $\cos \left(
360{}^\circ \times n+x \right)$ can be written as \[\cos \left( x \right)\].
Here, in this question we have n = 4 and x = 100.
Using this property, we can write the above expression as:
\[\cos 1540{}^\circ =\cos \left( 360{}^\circ \times 4+100{}^\circ \right)\]
\[\cos 1540{}^\circ =\cos 100{}^\circ \]
Now, we will come to the inverse part.
We know the property that for an angle x, if the measure of angle x is greater than or equal
to \[0{}^\circ \] and less than or equal to \[180{}^\circ \] , then the expression \[{{\cos }^{-
1}}\left( \cos x \right)\] can be written as x
i.e. \[{{\cos }^{-1}}\left( \cos x \right)=x\] for \[0{}^\circ \le x\le 180{}^\circ \]
Now, since in our question \[100{}^\circ \] satisfies the condition of being greater than or
equal to \[0{}^\circ \] and less than or equal to \[180{}^\circ \] , we can use the above
property on it.
We will use this property to calculate \[{{\cos }^{-1}}\left( \cos 1540{}^\circ \right)\]
\[{{\cos }^{-1}}\left( \cos 1540{}^\circ \right)={{\cos }^{-1}}\left( \cos 100{}^\circ \right)\]

\[{{\cos }^{-1}}\left( \cos 100{}^\circ \right)=100{}^\circ \]
Hence, \[{{\cos }^{-1}}\left( \cos 1540{}^\circ \right)=100{}^\circ \]
This is our final answer.

Note: In this question, it is very important to identify that the range of the function \[{{\cos}^{-1}}\left( x \right)\] is between \[0{}^\circ \] and \[180{}^\circ \]. In our question, we are given \[1540{}^\circ \] which is not in this range. So, we cannot directly write \[{{\cos }^{-1}}\left(\cos 1540{}^\circ \right)=1540{}^\circ \]. This would be wrong. So, we need to simplify \[1540{}^\circ \] to a smaller number such that it can be within the range.