∴CG=NG,CF=DG=NF,
∴S 1=(CG+DG)2
=CG2+DG2+2CG? By grace.
=GF2+2CG? DG,
S2=GF2,
S3=(NG-NF)2=NG2+NF2-2NG? NF,
∴S 1+S2+S3=GF2+2CG? DG+GF2+NG2+NF2-2NG? NF=3GF2= 16,
∴GF2= 163,
∴S2= 163.
So the answer is 163.