Sn=a1(1-q^n)/1-q=48
s2n=a1(1-q^2n)/1-q=60
那么s2n/sn=1+q^n=60/48
q^n=1/4
q^2n=1/16
q^3n=1/64
s3n=a1(1-q^3n)/1-q
s3n/sn=1+q^2n+q^n=1+1/4+1/16=21/16
s3n=sn*21/16=63