Tính
$cos\frac{\Pi }{2015}.cos\frac{2\Pi }{2015}.cos\frac{3\Pi }{2015}....cos\Pi$
ta luôn có cos(a)=-cos($\pi$-a)
=> A=-$(cos(\frac{\pi}{2015})........cos(\frac{1007\pi}{2015}))^2.cos(\pi)$
mặt khác ta có
$cos(\frac{\pi}{2015}).cos(\frac{2013\pi}{4030})=\frac{1}{2}(cos(\frac{\pi}{2})+cos(\frac{2011\pi}{4030}))=\frac{1}{2}cos(\frac{2011\pi}{4030})$
tương tự ta có
$cos(\frac{2\pi}{2015})cos(\frac{2011\pi}{4030})=\frac{1}{2}cos(\frac{2009\pi}{4030})$
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.
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$cos(\frac{1007\pi}{2015})cos(\frac{\pi}{4030})=\frac{1}{2}cos(\frac{2013\pi}{4030})$
nhân vế theo vế rồi rút gọn ta được A=$\frac{-1}{2^{2014}}.cos(\pi)$=$\frac{1}{2^{2014}}$
$sin\frac{\pi}{2015}A=sin\frac{\pi}{2015}.cos\frac{\pi}{2015}.cos\frac{2\pi}{2015}...cos\pi=\left ( \frac{1}{2} \right )^{2015}.cos2\pi\\ \Rightarrow A=\left ( \frac{1}{2} \right )^{2015}\dfrac{cos2\pi}{sin\dfrac{\pi}{2015}}=\frac{1}{2^{2015}}.\frac{1}{sin\frac{\pi}{2015}}$
bạn nhầm rồi đâu có ct nhân đôi như vậy