$21$.
$b)(x-2)(5-x)=0 $
$=>x-2=0 $ và $5-x=0 $
$x=0+2 $ $x=0+5 $
$x=2 $ $x=5 $
Vậy..........
$22.$
$30.(x+2)-6.(x-5)-24x=100$
$30.x+30.2-6.x-(-5.6)-24.x=100$
$30.x+60-6.x+30-24x=100$
$(30.x-6.x-24.x)+60+30=100$
$x.(30-6-24) = 100-60-30$
$x.0 = 10 $ (vô lí)
Vậy$ x = ∅$
$23. $
$125.(-61).(-2)^{3} .(-1)^{2n} $
$=125.(-61).(-8).1 $
$=[125.(-8)].(-61).1 $
$=-1000.(-61) $
$=61000 $
