求问两道matlab基础题

求问两道matlab基础题
1、求解方程组
2x+y+3z=13
x-y=-1
y+z=5
为什么这么写不对?：
>> A=[2,1,3;1,-1,0;0,1,1];
>> b=[13,-1,5]';
>> x=A\b
Warning:Matrix is singular to working precision.
x =
Inf
Inf
Inf
>>
2、在区间[-pi,pi]上,分别绘制sin(x)和cos(x)二条曲线.