i^(2)=-1
i^(3)=[blue]-i[/blue]
i^(4)=[b]1[/b]
цикл из четырех повторов
i^(15)=i^(12)*i^(3)=(i^(4))^(3)*[blue]i^3[/blue]=[b]1[/b]^3*[blue](-i)[/blue]=[blue]-i[/blue]
i^(713)=(i^(4))^(178)*i=1*i=i
i^(316)=(i^(4))^(79)=1
5*i^(713)+2*i^(316)-3*i^(15)+5i^2=5*i+2*1-3*(-i)+5*(-1)=-3+8*i