S2+1/S2+2=a2=s2-a1=s2+2/3
s2=-3/4
s3+1/s3+2=a3=s3-s2=s3+3/4
s3=-4/5
s4+1/s4+2=a4=s4-s3=s4+4/5
s4=-5/6
sn=-(n+1)/(n+2)