Răspuns:
[tex]a3 = (( - 1) ^{3} + ( - 1) ^{3 + 1}) \div 2 \\ a3 = ( - 1 + 1)2 \\ a3 = 0 [/tex]
[tex]a7 = ( ( - 1)^{7} + ( - 1)^{8} ) \div 2 \\ a7 = ( - 1 + 1) \div 2 \\ a7 = 0[/tex]
[tex]a12 = (( - 1)^{12} + ( - 1)^{13} ) \div 2 \\ a12 = ( 1 - 1) \div 2 \\ a12 = 0[/tex]
