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Q.

How do you evaluate cos(53π6)?

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a

32

b

31

c

52

d

-32 

answer is D.

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Detailed Solution

For general form of cos(x), we will convert the value of into the closest multiple of π2 and add or subtract a certain value α from that multiple of π2 to make it equal to .
Let’s assume that x=k× π2+α,  k  Z, |α|π2.  For the given angle 53π6, we will express it as
53π6=18×π2-π6.
The value of is even meaning that the trigonometric ratio remains cos.
The position of the angle will be in the second quadrant. The angle completes the half-circle 9 times and then will come back π6.
Hence, the sign becomes negative.
The final form will be  cos(53π6)=cos(18×π2-π6).
 cos(53π6)=-cos π6=-32
 
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