Q. 403.7( 3 Votes )

Find the value off the following:

(sin 72° + cos 18°)( sin 72° - cos 18°)

Answer :

(sin 72° + cos 18°)( sin 72° - cos 18°)

Using the identity , (a-b)(a+b) = a2 – b2


= (sin 72°)2 - (cos 18°)2


= {cos(90° - 72°)}2 - (cos 18°)2 [ Sin θ = cos (90° - θ)]


=(cos 18°)2 - (cos 18°)2


= 0


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