Question

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cos 7π/20 cos 3π/20-sin π/25 sin 4π/15
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Answer 1

$cosfrac{7pi}{20}*cosfrac{3pi}{20}-sinfrac{pi}{15}*sinfrac{4pi}{15}\\ cosfrac{7pi}{20}*cosfrac{3pi}{20} = frac{cosfrac{pi}{2}+cosfrac{pi}{5}}{2}\\ sinfrac{pi}{15}*sinfrac{4pi}{15}=frac{cosfrac{pi}{5}-cosfrac{pi}{3}}{2}\\ frac{cosfrac{pi}{5}-cosfrac{pi}{5}+cosfrac{pi}{3}}{2}=frac{frac{1}{2}}{2}=frac{1}{4}$