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| 1. |
Sin a.sin (60 - a) sin a( 60 + a)=1/4sin3a |
| Answer» sinA . sin(60 – A) . sin(60 + A) = sinA . (sin60 cos A – cos60 sin A) . (sin60 cos A + cos60 sin A)= sinA . (sin260 cos2A – cos260 sin2A) = sinA . {(3/4) cos2A – (1/4) sin2A)= sinA . {(3/4) – (3/4) sin2A – (1/4) sin2A)= sinA .{(3/4) – sin2A) = (3/4) sinA – sin3A= 1/4 (3 sinA – 4sin3A) =1/4 sin3A | |