y=anx^2
y'=2anx
x=1
y'=2a(n-1)+1
2an=2a(n-1)+1
2[an-a(n-1)]=1
an-a(n-1)=1/2
即d=1/2
n=1时过(2,8)
8=a1*2^2
a1=2
a7=a1+6d
=2+6*(1/2)
=5
y=anx^2
y'=2anx
x=1
y'=2a(n-1)+1
2an=2a(n-1)+1
2[an-a(n-1)]=1
an-a(n-1)=1/2
即d=1/2
n=1时过(2,8)
8=a1*2^2
a1=2
a7=a1+6d
=2+6*(1/2)
=5