3a(2)+a(4)=20
3(a(1)+d))+a(1)+3d=20
4a(1)+6d=20
2a1=10-3d
a1=5-1.5d
f(d)=a(3)*a(5)=(a(1)+2d)(a(1)+4d)=
подставлю выражение а1
=(5-1.5d+2d)(5-1.5d+4d)=(5+0.5d)(5+2.5d)=25+12.5d+0.25d+1.25d^2=
=25+15d+1.25d^2
найду вершину параболы
d0=-15/2.5=-6
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3a(2)+a(4)=20
3(a(1)+d))+a(1)+3d=20
4a(1)+6d=20
2a1=10-3d
a1=5-1.5d
f(d)=a(3)*a(5)=(a(1)+2d)(a(1)+4d)=
подставлю выражение а1
=(5-1.5d+2d)(5-1.5d+4d)=(5+0.5d)(5+2.5d)=25+12.5d+0.25d+1.25d^2=
=25+15d+1.25d^2
найду вершину параболы
d0=-15/2.5=-6