1. (√2-1)(x+1)=0
х+1=0
х=-1
2. 0.2(x-5)+0.8(1-x)-2.4(2x+5)=4
0,2х-1+0,8-0,8х-4,8х-12=4
-5,4х=4+1-0,8+12=16,2
х=16,2:(-5,4)=-3
3. 2x(x-2)+3x²-5(x²+6x+1)=12
2х²-4х+3х²-5х²-30х-5=12
-34х=17
х=-17/34=-0,5
4. 3(x-12)-4x·0.75=-36
3х-36-3х=-36
0х=0
х∈(-∞;∞)