18. a) 2x-4<=10
2x<=14
x<=7
b) 6x+24-24<=12
6x<=12
x<=2
c) x-5>1296
x>1301
x>7×2
x>14
d) x<=27:3
x<=9
x>=12:4
x=>3
e)
[(x-12)×3-2]<=4×7
3x-36-2<=28
3x-36<=28+2
3x<=30+36
3x<=66
x<=22
f) 3x>=23+4
3x>=27
x>=9
19.
a) 3x-6<=3
3x<=9
x<=3
b) (x+2)×3<=60
x+2<=20
x<=18
c) 18x-14>=6x+10
18x-6x>=10+14
12x>=24
x>=2
d) x-4>9×243
x-4>2187
x>2191
x>=5×2
x>=10
e)2x+3x<=5×8
5x<=40
x<=8
f) 4×18x<=36×30
4×18x<=1080
18x<=1080:4
18x<=270
x<=270:18
x<=15
