1
x²-6x+8≥0
x1+x2=6 U x1*x2=8
x1=2 U x2=4
x∈(-∞;2] U [4;∞)
2
(4-x²)/(x+1)>0
(2-x)(2+x)/(x+1)>0
x=2 x=-2 x=-1
+ _ + _
---------------(2)----------------(-1)--------------(2)-------------------
x∈(-∞;-2) U (-1;2)
3
{(25-x²)≥0⇒(5-x)(5+x)≥0⇒-5≤x≤5
{x+3>0⇒x>-3
{log(21)(x+3)≠0⇒x+3≠1⇒x≠-2
x∈(-3;-2) U (-2;5]