Question : If $\cos3A = \sin (A-18°)$, where 3A is an acute angle, what is the value of A?
Option 1: 110°
Option 2: 108°
Option 3: 54°
Option 4: 27°
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Correct Answer: 27°
Solution :
Given, $\cos3A = \sin (A-18°)$
We know, $\cos A = \sin(90°-A)$
⇒ $\sin(90°-3A)=\sin(A-18°)$
⇒ $90°-3A=A-18°$
⇒ $90°+18 = A+3A$
⇒ $4A=108°$
⇒ $A=27°$
Hence, the correct answer is 27°.
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