三大题，任答一题都好啊?高三文科数学

a1=1,a(n+1)-an=4/[a(n+1)+an]

[a(n+1)]^2-[an]^2=4

{[an]^2}是等差数列,公差=4

[an]^2=(a1)^2+4(n-1)=4n-3

an=√(4n-3)

2)1/[a(n+1)+an]=1/[√(4n+1)+√(4n-3)]=1/4[√(4n+1)-√(4n-3)]

Sn=1/4[(√5-1)+(3-√5)+(√13-3)+(√17-√13)+(√21-√17)+...√(4n+1)-√(4n-3)]=1/4[1+√(4n+1)]