确定a,b的值使线性方程组有解,并求其解：（1）2x1-x2+x3+x4=1,x1+2x2-x3+x4=2,x1+7x2-4x3+11x4=a (2)x1+2

2 -1111
12 -112
17 -4 11a
r1-2r2, r3-r2
0 -53 -1 -3
12 -112
05 -3 10a-2
r3+r1
0 -53 -1 -3
12 -112
0009a-5
r1*(-1/5), r3*(1/9)
01 -3/51/53/5
12-112
00 01a/9-5/9
r1+(1/5)r3, r2-r3
01 -3/5032/45 - a/45
12-1 023/9 - a/9
00 0 1 a/9 - 5/9
r2-2r1
01 -3/5032/45 - a/45
101/5017/45 - a/45
00 0 1 a/9 - 5/9
a 取任何值方程组都有解
通解为: (17/45 - a/45, 32/45 - a/45, 0, a/9 - 5/9)' + c(-1/5,3/5,1,0)'.