假设X(1)=x,X(n)=xk^(n-1) (k>0,x>0,x≠1)
y4=2log a X4=2log a(xk^3)=2loga x+2loga k^3=2loga x+6loga k=17
y7=2log a X7=2log a(xk^6)=2loga x+2loga k^6=2loga x+12loga k=11
所以6loga k=-6
loga k=-1
2loga x =23
yn=2log a Xn =2log a [xk^(n-1)]
=2loga x + (n-1)2log a k
=2loga x -(2n-2)
=23-2n+2
=25-2n
yn-y(n-1)=25-2n-25+2(n-1)
=-2n+2n-2
=-2
所以等差,公差-2
(2) ym=0
25-2m=0
m=25/2
所以y13