A*(bc-a)+b(ac-b)+c(ab-c) =abc-a²+abc-b²+abc-c²=3abc-(a²+b²+c²)=
=3abc-0=3abc
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(2x²+x-1)²=
разожим на множители 2x²+x-1=0
D=1+8=9
x=(-1+3)/4=1/2
x=(-1-3)/4= -1
2*(x- (1/2))(x+1)=(2x-1)(x+1)
(2x²+x-1)²=((2x-1)(x+1))²=(2x-1)²(x+1)²=(4x²-4x+1)(x²+2x+1)=
=4x⁴+8x³+4x²-4x³-8x²-4x+x²+2x+1=4x⁴+4x³-3x²-2x+1
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a²+b² a+b=7 ab=12 (a+b)²=a²+2ab+b² ⇒a²+b²=(a+b)²-2ab
7²-2*12=49-24=25