数列bn=2^n/(4^n-1),证明b1+b2+b3+……+bn

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数列bn=2^n/(4^n-1),证明b1+b2+b3+……+bn
数列bn=2^n/(4^n-1),证明b1+b2+b3+……+bn

数列bn=2^n/(4^n-1),证明b1+b2+b3+……+bn
bn=2^n/(4^n-1)
b1= 2/3
b2 = 4/15
b3 = 8/63
for n>=4
bn =2^n/(4^n-1)
< (2^n +1)/(4^n -1)
= (2^n +1)/[(2^n -1)(2^n +1)]
= 1/(2^n -1)
< 1/2^(n-1)
Sn = b1+b2+...+bn
= 2/3 +4/15+8/63+ (b3+b4+...+bn)
< 2/3 +4/15 +8/63+ [1/2^3+1/2^4+...+1/2^(n-4) ]
= 2/3 + 4/15 +8/63+ (1/4)( 1- (1/2)^(n-3))
< 2/3 + 4/15 + 8/63+1/4
= (630+336+160+315)/1260
=1441/1260

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