[tex]a)13=9+4=3^{2}+2^{2}[/tex]
[tex]b)13^{31}=13*13^{30}=(3^{2}+2^{2})*13^{30}=3^{2}*13^{30}+2^{2}*13^{30}=[/tex]
[tex]3^{2}*(13^{15})^{2}+2^{2}*(13^{15})^{2}=(3*13^{15})^{2}+(2*13^{15})^{2}[/tex]
[tex]c)65=64+1=8^{2}+1^{2}[/tex]
[tex]d)45^{45}=45*45^{44}=(36+9)*45^{44}=(6^{2}+3^{2})*45^{44}=[/tex]
[tex]6^{2}*45^{44}+3^{2}*45^{44}=6^{2}*(45^{22})^{2}+3^{2}*(45^{22})^{2}=[/tex]
[tex](6*45^{22})^{2}+(3*45^{22})^{2}[/tex]