(1)
a2²+a3²=a4²+a5²
a2²-a5²=a4²-a3²
(a2+a5)*(a2-a5)=(a4+a3)*(a4-a3)
-3d*(a4+a3)=d*(a4+a3)
∵d≠0 ∴a4+a3=0
又S7=(a1+a7)*7/2=2a4*7/2=a4*7=7
故a4=7/7=1 a3=-1 d=2
an=1+2*(n-4)=2n-7
(2)
f(m)=ama(m+1)/a(m+2)=(2m-7)*(2m-5)/(2m-3)=2m-9+8/(2m-3)
an>=a1=-5,所以an為任何大于等于-5的奇數(shù)
8/(2m-3)必為偶數(shù),且考慮2m-3為奇數(shù),那么2m-3的取值只有為1或-1
2m-3=-1（m=1）時(shí),f(m)=2-9-8