P1 := y-x-z=1; P2:= x+y-z=-1;   Line := solve({P1,P2},{x,y,z});  theta := convert(angle([-1,1,-1],[1,1,-1]),degrees);  evalf(",3);  Ang := proc(v1,v2) evalf(convert(angle(v1,v2),degrees),3); end;  t1 := Ang([2,1,0],[-1,1,1]); t2 :=Ang([2,1,0],[1,0,1]); t2 :=Ang([2,1,0],[-2,2,1]);  volume := innerprod([1,-1,2],crossprod([1,1,-1],[3,1,0]));  p := vector([0,1,0]); q := vector([2,1,0]); r := vector([1,0,1]);  A := crossprod(q-p,r-q)/2; Area := sqrt(innerprod(A,A));  Line := [0,0,0] + t*[-1,-1,1];  symLine := { x=y,y=-z};  L := {x=-3*t-1, y=2*t-2, z=t-1}; P := {x+y+z=3};  solve(L union P,{x,y,z,t});  solve(L union P,t);