【数学】求详细解答!!谢谢!!!最好发图片来!!!!!
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Sn=na1+n(n-1),
S2=2a1+2,S4=4a1+12,
S1,S2,S4成等比数列,
∴(2a1+2)^2=a1(4a1+12),
8a1+4=12a1,a1=1,
(I)an=2n-1.
(II)bn=(-1)^(n-1)*4n/[(2n-1)(2n+1)]
=(-1)^(n-1)*[1/(2n-1)+1/(2n+1)],
∴Tn=(1+1/3)-(1/3+1/5)+(1/5+1/7)+……+(-1)^(n-1)*[1/(2n-1)+1/(2n+1)]
={1-1/(2n+1),n为偶数;
{1+1/(2n+1),n为奇数
=1+[(-1)^(n-1)]/(2n+1).
S2=2a1+2,S4=4a1+12,
S1,S2,S4成等比数列,
∴(2a1+2)^2=a1(4a1+12),
8a1+4=12a1,a1=1,
(I)an=2n-1.
(II)bn=(-1)^(n-1)*4n/[(2n-1)(2n+1)]
=(-1)^(n-1)*[1/(2n-1)+1/(2n+1)],
∴Tn=(1+1/3)-(1/3+1/5)+(1/5+1/7)+……+(-1)^(n-1)*[1/(2n-1)+1/(2n+1)]
={1-1/(2n+1),n为偶数;
{1+1/(2n+1),n为奇数
=1+[(-1)^(n-1)]/(2n+1).
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