已知数列{an}中,a1=2,a2=4,a(n+2)=a(n+1)-an,则a10等于?
问题描述:
已知数列{an}中,a1=2,a2=4,a(n+2)=a(n+1)-an,则a10等于?
答
由a(n+2)=a(n+1)-an,和a(n+3)=a(n+2)-a(n+1)
得an+a(n+3)=0,
所以an+a(n+9)=0,即有a1+a10=0,
所以a10=-2