帮我解这些方程啊.一个过程就行.
1.4x+y-z=12 3x+2y+z=-5 x-y+5z=1
2.2a+7b=3 3a-c=1 -b+3c=4
3.x:y=3:2 y:Z=5:4 x+y+z=66
4.a-d=-4 d-2y=-1 a+s-d=-1
5.a=6s a-4=10s-20d a+12=3y+18d
6.a+s=8 a+d=6 s+d=10
7.3(a+s-d)=15(d-a) 5(a+s-d)=400 15+53/6(a+s)=10000
8.2a+s+d=15 a+2s+d=16 a+s+2d=17
9.2a=3s=6d a+2s+d=16
10.A:S:D=3:4:5 A+S+D=36
11.3x-y+z=3,2x+y-3z=11,x+y+z=12
12.5x-4y+4z=13,2x+7y-3z=19,3x+2y-z=18
13.a+b+c=-4,49a+7b+c=8,25a+5b+c=0
14.x+y/2=z+x/3=y+z/4,x+y+z=27
15.X=2Y,2X+Y+2Z=1,2X-Z=7,
16.Y-2Z=4,Y-3Z=10； 3X+Y=9.
17.x-2y-3z+18=0,x+3y-2z-8=0,x+y+2z-24=0
18.90a+30b+c=300,160a+40b+c=150,250a+50b+c=100
19.x:y=1:5,y:z=2:3,x+y+z=27
20.x+y=3,2x-y+2z=2,x-y-z=-3