Q. 3 H4.2( 5 Votes )

Prove that :

sin 51° + cos 81° = cos 21°

Answer :

Take L.H.S:


sin 51° + cos 81°


= sin 51° + sin (90° - 81°)


{sin (90° - A) = cos A}


= sin 51° + sin 9°





= 2 sin 30° cos 21°



= cos 21°


= R.H.S


Hence Proved


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