Q. 5 F4.0( 4 Votes )

# Prove that:</

Answer :

Take L.H.S

tan 20° tan 40° tan 30° tan 80° Multiplying & Dividing by 2: { 2 sin A sin B = cos (A – B) – cos (A + B) &

2 cos A cos B = cos (A + B) + cos (A – B)}  { cos (-A) = cos A}   { 2 cos A sin B = sin (A + B) – sin (A – B) &

2 cos A cos B = cos (A + B) + cos (A – B)}  { sin (-A) = - sin A}  { sin (180° - A) = sin A & cos (180° - A) = - cos A}  = 1

= R.H.S

Hence Proved

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