Step 1: Read the given question
GIVEN: \cos 36\degree \cos 54\degree - \sin 36\degree \sin 54\degree
Step 2: Using the trigonometric ratios of complimentary angles.
NOTE: \cos A = \sin (90\degree - A)
EXAMPLE: \cos 36\degree = \sin(90\degree - 36\degree)
\cos 36\degree = \sin(54\degree)
\cos 54\degree = \sin(90\degree - 54\degree)
\cos 54\degree = \sin(36\degree)
Step 3: Substitute the calculated values in the given equation
NOTE: = \cos 36\degree \cos 54\degree - \sin 36\degree \sin 54\degree
= \sin 54\degree \sin 36\degree - \sin 36\degree \sin 54\degree (since step 2)
= 0
