Solution :
The expression = (sin78 – sin42) – (sin66 – sin6)
= 2cos(60)sin(18) – 2cos36.sin30
= sin18 – cos36
= \(({\sqrt{5} – 1\over 4})\) – \(({\sqrt{5} + 1\over 4})\) = -\(1\over 2\)
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