f(x) = x^4+1

according Viete rule:

We know that x1+x2+x3+x4=-b/a= 0

Then S=0

(x1+x2+x3+x4)^2 = (x1+x2+x3+x4)(x1+x2+x3+x4)

Open brackets:

S^2 =x1^2+x1*x2+x1*x3+x1*x4+x2*x1+x2^2+x2*x3+x2*x4

+x3*x1+x3*x2+x3^2+x3*x4+x4*x1+x4*x2+x4*x3+x4^2

= (x1^2+x2^2+x3^2+x4^2)+ 2(x1*x2+x1*x3+x1*x4+x2*x3+x2*x4+x3+x4)

But x1*x2+x1*x3+x1*x4+x2*x3+x2*x4+x3+x4= c/a =0/1=0

= T + 2(c/a)

==> T= s^2 - 2(c/a) = 0-0=0

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