海之舟
幼苗
共回答了22个问题采纳率:81.8% 举报
an=n^2(cos(n60^2-sinn60^2)=n^2cos(2nπ/3),最小正周期2nπ/3=2π,n=3,cos(2nπ/3)的值{-1/2,-1/2,1,-1/2,-1/2,1┄┄┄},把一个周期看成一组,每组和的通项=-(3n-2)²/2-(3n-1)²/2+(3n)²=(18n-5)/2,组与组之间为等差数列d=18/2=9,S30=[(18n-5)/2+(30/3-1)*9]*10/2=[13/2+162/2]*5=875/2
1年前
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