Correct Answer - Option 1 : √2 sin27°

**Concept:**

cos (90 - x) = sin x

sin A - sin B = 2cos\(\rm (A+B) \over 2\)sin \(\rm (A-B) \over 2\)

cos 45° = \(1 \over \sqrt 2\)

__Calculation:__

Given,

cos18° - sin18°

= cos (90° - 72°) - sin 18°

= sin 72° - sin 18°

= 2 cos \(\rm (72°+18°) \over 2\)× sin \(\rm (72°-18°) \over 2\)

= 2 cos 45° sin 27°

= 2× \(1 \over \sqrt 2\)× sin 27°

= √2 sin 27°