Bài 7:
a) có 2 trg hợp :
th1 : x - 17=-10+38 => x-17+10-38 = 0
=> x - 45 = 0 => x=45
th2: -x-17=-10+38 => -x-17+10-38=0
=> -x-45=0=>x+45=0=>x=-45
=> x= {45; -45}
b)có 2 trg hợp:
th1 : x+7--8=14=>x+7+8-14=0=>x+1=0=>x=1
th2:-x-7-8=14=>x+7-8-14=0=>x-15=0=>x=15
=>x={1;15}
c) x ∈ B(4) =>B(4) = 0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40 mà -2<x<6
=> x∈{0;4}
d)x∈B(6) =>B(6)=: 0, 6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66,... mà 0<x<6
=>x∈(∅}
e)-x/12=y/3=40/6 => -x/12=4y/12=80/12=>theo dãy tỉ số bằng nhau : -x+4y+80/12+12+12
g)3/x=12/8=>3.8=12x=>24=12x=>x=24:12=2
l)2/x=x/8=>2.8=x²=>16=x²=>x=4
m)3x-40%=-1,2=>3x-0,4=-1,2=>3x=-1,2+0,4=>3x=-0,8=>x=-4/15
n)4/7 : x=13 => 4/7*1/x=13=>4/7x=13=>4/7x - 13=0
