作业帮·求tn=?Sn=75/4(5^n-1)
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·求tn=?Sn=75/4(5^n-1)
·求tn=?Sn=75/4(5^n-1)
·求tn=?Sn=75/4(5^n-1)
Sn = a(1-r^n) / (1-r)
= 75*(1-5^n)/(-4)
= 75* (1-5^n) / (1-5)
a = 75
r = 5
tn = 75*5^(n-1)
·求tn=?Sn=75/4(5^n-1)
若 Sn=75/4(5^n-1) tn=?答案是75（5^n-1).
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