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Find the value of 2cos(pi/(10))cos((9pi)...

Find the value of `2cos(pi/(10))cos((9pi)/10)+cos((2pi)/10)`

A

1

B

2

C

-1

D

None of these

Text Solution

Verified by Experts

The correct Answer is:
C
`{(1 / √2) [cos (9π / 10) – sin (9π / 10)]} = cos [(π / 4) + (9π / 10)]`

=` cos (23π / 20)`

=` cos [(2π) – (23π / 20)]`

= `cos 17π / 20`

...
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