a(n)=a+(n-1)d,s(n)=na+n(n-1)d/2.b=a(2)=a+d,a(1)=a=b-d,a(3)=a+2d=b+d,-3=a(1)+a(2)+a(3)=b-d+b+b+d=3b,b=-1.8=a(1)a(2)a(3)=(b-d)b(b+d)=b(b^2-d^2)=-(1-d^2),d^2=9,d=3或d=-3.[a(3)]^2=(b+d)^2=(-1+d)^2=a(1)a(2...