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Q.

From the following options, find the value of sin 18 ° cos 72 °  .


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a

sin72°  

b

cos18°  

c

1

d

0 

answer is C.

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Detailed Solution

Given that, sin 18 ° cos 72 °  .
We know that, the complementary formula for sinθ   is:
sin( 90 ° −θ)=cosθ  
Convert sinθ   into cosθ   trigonometric function.
⇒ sin 18 ° cos 72 ° = sin( 90 ° − 72 ° ) cos 72 °   Apply complementary angle formula,
  ⇒ sin 18 ° cos 72 ° = cos 72 ° cos 72 ° ⇒ sin 18 ° cos 72 ° =1   Thus, sin 18 ° cos 72 ° =1  .
Hence, option 3) is correct.
 
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