Теорема виета для x^2+px+q
x1+x2=-p
x1x2=q
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x^2+3x-9=0
x1+x2=-3
x1x2=-9
x1²+x2²=(x1+x2)²-2x1x2=(-3)²-2*(-9)=9+18=27
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x^2+3x-9=0
1/x1 1/x2
(x-1/x1)(x-1/x2)=x^2-1/x1*x-1/x2*x+1/x1x2=x^2-(x1+x2)/x1x2*x+1/x1x2=x^2-(-9)/(-3)*x-1/(-9)=
=x²-3x+1/9