커뮤니티

∑{(-1/3)^n - (1/3)^n cos^2 (nㅠ/3)}= ∑{(-1/3)^n -∑ (1/3)^n cos^2 (nㅠ/3)}
= ( - 1/3)/ {1+ 1/3 } - { (1/3)* (1/4)+(1/3)^2 *(1/4) +(1/3)^3 +(1/3)^4* (1/4)+(1/3)^5 *(1/4) +(1/3)^6 +...}= - 1/4 -{가*(1/4)+나*(1/4)+다}

가=초항 1/3 공비 (1/3)^3 = (1/3)/{1-(1/27)} = 9/26
나=초항 (1/3)^2 공비 (1/3)^3 = 3/26
다=초항 (1/3)^3 공비 (1/3)^3 =1/26
그래서 준식= -1/4 -{9/104 + 3/104+4/104}= -1/4 - 16/104
= -1/4 -4/26= -13/52 - 8/52= -21/52
답이 이상하네요.
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