已知数列{an}中,a1=1,a2=0,对任意正整数n,m(n>m)满足a2n−a2m=an−man+m,则a119=_.
问题描述:
已知数列{an}中,a1=1,a2=0,对任意正整数n,m(n>m)满足
−
a
2n
=an−man+m,则a119=______.
a
2m
答
令n=2,m=1,则(a2)2-(a1)2=a1a3;
因为a1=1,a2=0,所以a3=-1;
令n>2,m=2,则(an)2-(a2)2=an-2an+2,
所以
=an+2 an
;an an−2
所以
=a119 a117
=…=a117 a115
=-1a3 a1
所以
=(a119 a1
)59=-1;a3 a1
所以a119=-a1=-1