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Question : 14 of 120
Marks:
+1,
-0
Solution:
S = sin 20° sin 40° sin 80°
⇒S=‌sin‌40°(2‌sin‌80°‌sin‌20°) ⇒S=‌sin‌40°[cos(80−20)−cos(80+20)] ⇒S=‌sin‌40°[cos‌60−cos‌100] ⇒S=‌sin‌40°−‌sin‌40‌cos‌100 ⇒S=‌sin‌40°−(2‌sin‌40‌cos‌100) ⇒S=‌sin‌40°−[sin(100+40) −sin(100−40)] ⇒S=‌sin‌40°−[sin‌140−sin‌60] ⇒S=‌sin‌40°−‌sin‌140+‌sin‌60 ⇒(sin‌40°−sin‌140)+ ⇒S=[sin‌40°−sin(180−40)]+ ∵sin(180−x)=sin‌x ⇒S=(sin‌40°−sin‌40)+ ⇒S=
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