Thanks everybody for the useful prompt replies.
Just to point out what I was aiming at, here follows the list of instructions
I intended to suggest to my pupils in order to show them the influence of the
parameter K when linearly combining two lines r and s into a star (ex:r+K*s).
Namely, varying K from 0 to a possibly high value, the expression ex
approaches nearer and nearer the line s.
(%i6) a:1;
(%i7) b:1;
(%i8) c:1;
(%i9) e:1;
(%i10) f:0.5;
(%i11) g:1;
(%i12) r:a*x+b*y+c;
(%i13) s:e*x+f*y+g;
(%i14) P:linsolve([r,s],[x,y]); centre point of the star
(%i15) ex:r+K*s;
(%i16) solve(ex,y); copy and paste the rhs of the output as rhs of %i17
(%i17) Z(x,K):=-((2*x+2)*K+2*x+2)/(K+2);
(%i18) F:makelist(Z(x,K),K,0,10);
(%i19) plot2d(F,[x,-5,5]);
One thing I haven't been able to do is to avoiding copying-pasting the rhs
output of %i16
y=-((2*x+2)*K+2*x+2)/(K+2)
as the rhs in %i17. I tried with:
Z(ex,K):=solve(ex,y);
but that didn't work.
Thanks,
Pier