Krishna
0

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