6)
x²-4y²+2x-12y-5=x²+2x+1-1-(2y)²-12y-9+9-5=(x+1)²-((2y)²+2*2y*3+3²)-1+9-5=
=(x+1)²-(2y+3)²+3=(x+1+2y+3)(x+1-2y-3)+3=(x+2y+4)(x-2y-2)+3.
8)
y=1/(1+y/x) z=1/(1+z/y) z=2 x-?
2=1/(1+2/y)
2=1/((y+2)/y)
2=y/(y+2)
y=2*(y+2)
y=2y+4
y=-4
-4=1/(1+(-4)/x)
-4=1/((x-4)/x)
-4=x/(x-4)
-4*(x-4)=x
-4x+16=x
5x=16 |÷5
x=16/5=3,2.
10)
a²+b²+c²+3≥2*(a+b+c)
a²+b²+c²+3≥2a+2b+2c
a²+b²+c²+1+1+1-2a-2b-2c≥0
(a²-2a+1)+(b²-2b+1)+(c²-2c+1)≥0
(a-1)²+(b-1)²+(c-1)²≡≥0.