用代入法解下列方程组(急用,请快点.先给5分,答完再给50分)

1.
y=x+3.......(1)
7x+5y=9.....(2)
解(1)代入(2),
7x+5(x+3)=9
7x+5x+15=9
解得,
x=-1/2
y=(-1/2)+3=5/2

2.
3s-t=5.......(1)
5s+2t=15.....(2)
(1)*2+(2)得,
11s=25,t=25/11
将t=25/11代入(1)
3s-25/11=5
s=(5-25/11)/3=10/11

3.
3x+4y=16......(1)
5x-6y=33......(2)
(1)*3+(2)*2得,
19x=16*3+33*2=114
x=114/19=6
将x=6代入(1),
3*6+4y=16
y=-1/2

4.
4(x-y-1)=3(1-y)-2....(1)
x/2+y/3=2............(2)
整理(1)得,
4x-y=5...............(3)
整理(2)得,
3x+2y=12.............(4)
(3)*2+(4)得,
11x=22,x=2
将x=2代入(3),
4*2-y=5,y=3