mathics
1.

Solve[{2x-2y+z-w==7,-x+4y-5z+3w==1,x+y-z+w==4,-4x+2y+z-2w==6},{x,y,z,w}]

2.

Solve[{x^3-21*x+20==0},{x}]

3.

Plot[ x^2-2*x+3, {x , 0 , 15}]

4.

Plot[ Cos[2*x], {x,-1,1}]

5.
Plot[Sin[1/x], {x,1,3}]
6.

7.

octave
1. 
A = [2 -2 1 -1; -1 4 -5 3;1 1 -1 1;-4 2 1 -2];
f = [7 1 4 6];
u = A\f'
2.
c = [1 0 -21 20];
roots(c)


3.
x=[-3:1:10];
y=x^2-2*x+3;
plot(x,y,".c")
4.
x=[0:0.1:10]
y=cos(2*x)
plot(x,y,"-g")



5.

x=[-3:0.1:10];
y=sin(x.^(-1));
plot(x,y,"-y")
6.
x = -2.0:1:2;
y = -2.0:1:2;
[X,Y]=meshgrid(x,y);
mesh(X,Y,X.^2+Y.^2)

7.
x=-2:1:2;
octave:67> y=-2:1:2;
octave:68> [X,Y]=meshgrid(x,y);
octave:69> mesh(X,Y,X.^2-Y.^2+2*X*Y.^2+1)