eqn1:solve(v_out=A*(v_p+e_n-v_n),v_out)[1]$
eqn2:solve(i1+i2+i_n-v_n/R1+(v_out-v_n)/R2=0,v_n)[1]$
eqn3:solve(i_n+i3+v_p/R3=0,v_p)[1]$
eqn1:subst(eqn2,eqn1)$
eqn1:subst(eqn3,eqn1)$
eqn1:solve(eqn1,v_out)[1]$
fac:(R2+(A+1)*R1)/(A*R2+A*R1)$
eqn1*fac$
tem1:expand(RHS(eqn1*fac))$
c0:ratsimp((coeff(tem1,e_n)));
c1:ratsimp((coeff(tem1,i_n)));
c2:ratsimp((coeff(tem1,i1)));
c3:ratsimp((coeff(tem1,i2)));
c4:ratsimp((coeff(tem1,i3)));