解
1)Y1-2*Y2=5
即 2X+5-2(3-4X)=5
2X-6+8X=0
X=6/10
X=3/5
2)-Y1+1/2*Y2=-1
即 -(2X+5)+1/2(3-4X)=-1
-2X-5+3/2-2X=-1
4X=-5/2
X=-5/8